Aperture priority combined with Auto ISO and Zoom lens seems odd

[pschlute]

but have found that Auto-ISO works well when shooting conditions have highly variable lighting conditions. When shooting at outdoor events where the action moves from full sunshine to full shadow, Auto-ISO works well.

If I am taking "action pictures" of my dogs outside i use Auto-ISO too. My camera has a TAv mode (basically Manual with auto-ISO). So i can set the shutter speed that freezes the action sufficiently, and set the aperture that gives me the required DOF, and let the camera sort out the exposure. Very handy when they run from full sun to shade.

[xpatUSA]

I'm confused now as two different calculators tell me its 14.67/15 EV as per sunny 16. What am I missing, f/22, 1/500s and ISO 800, no doubt something obvious?

Firstly, the formal definition from here:

https://en.wikipedia.org/wiki/Exposure_value

EV = log2(N^2/t) = log2(22^2/(1/500)) = 17.88 EV

Or the table here:

https://en.wikipedia.org/wiki/Exposu...amera_settings

Where f/22, 1/500 sec shows as 18 EV.

Any links to your two different calculators, Grahame?

[Stagecoach]

Firstly, the formal definition from here:

https://en.wikipedia.org/wiki/Exposure_value

EV = log2(N^2/t) = log2(22^2/(1/500)) = 17.88 EV

Or the table here:

https://en.wikipedia.org/wiki/Exposu...amera_settings

Where f/22, 1/500 sec shows as 18 EV.

Any links to your two different calculators, Grahame?

I'm not really interested in formal definitions or formula Ted, just knowing that the ops setting of f/22, 1/500s, ISO800 makes sense for obtaining a reasonable exposure on a 'sunny day'. (disregarding of course the f/22 choice)

Here's a couple I keep on my 'favourites'.

Exposure Comparison here - https://www.scantips.com/lights/exposurecalc.html

Exposure Calculator here - http://endoflow.com/exposure/

[xpatUSA]

I'm not really interested in formal definitions or formula Ted

Fine.

[dem]

The exposure the camera chose (1/500 sec for f/22) does not match "normal daylight"; see https://en.wikipedia.org/wiki/Exposure_value where I'd say normal daylight is about 14 EV. But f/22 and 1/500 sec work out to almost 18 EV causing your image to be 4 EV under-exposed! I don't know your camera so I can't offer why it did that but you are quite right in thinking there's something wrong - badly wrong, IMHO.

Ted, it all adds up. The 18 EV you calculated is the light value for ISO 100. The actual light value is 3 stops lower, that's why the camera uses ISO 800 rather than ISO 100.

To the OP, both my Fujis (a bridge camera with a crop factor of 4 and an interchangeable lens camera with a crop factor of 1.5) adjust the shutter speed so it is close to 1/full_frame_equivalent_focal_length. This formula does not take into account any image stabilisation and assumes "normal viewing conditions". If you crop your images and/or print them large, you will need faster shutter speeds. If you have a good image stabilisation or make small images for web, you can get away with much longer shutter speeds.

Test your equipment yourself to know the actual limits.

As has been mentioned you probably do not want to shoot at f/22.

[xpatUSA]

Ted, it all adds up. The 18 EV you calculated is the light value for ISO 100. The actual light value is 3 stops lower, that's why the camera uses ISO 800 rather than ISO 100.

It could well be - thanks. I'm never too clear about anything auto; being always in manual myself. I regard the final aperture and shutter settings as being the sensor exposure - with the ISO setting being a multiplier downstream of the sensor output. That is to say that the ISO setting does not affect the actual sensor exposure directly, only via the metering which in turn sets the aperture and/or shutter.

So the ISO 800 under-exposes the sensor indirectly via metering and that under-exposure is brightened pro rata by the AFE or during conversion (Sigma). The under-exposure is not unlike exposure compensation except that exposure compensation results in an appropriately darkened or brightened conversion - whereas auto-ISO results in the so-called average mid-gray conversion.

I was thinking about that yesterday after Grahame lost interest. Thanks again for the clarification.

[Stagecoach]

It could well be - thanks. I'm never too clear about anything auto; being always in manual, I regard the final aperture and shutter settings as being the sensor exposure - with the ISO setting being a multiplier downstream of the sensor output. That is to say that the ISO setting does not affect the actual sensor exposure directly, only via the metering which in turn sets the aperture and/or shutter.

I was thinking about that yesterday after Grahame lost interest.

It's not a case of "lost interest" Ted it's that I do not see the relevance to the posts situation, his description and want for an answer as to what's happening.

[dem]

Ted, I find it helpful to separate the light value (how bright the subject is) from the exposure value (what camera settings are used at a given ISO). A quick google search returns a link to Ken Rockwell's site:

https://kenrockwell.com/tech/ev.htm

which describes exactly how I would describe it, unlike the Wikipedia article which seems to use EV for LV and does not explain its connection to the ISO. Good old Ken.

Another good link:

https://www.scantips.com/lights/evchart.html

[xpatUSA]

It's not a case of "lost interest" Ted it's that I do not see the relevance to the posts situation, his description and want for an answer as to what's happening.

Thank you for clarifying "I'm not really interested in formal definitions or formula" Grahame.

[xpatUSA]

Ted, I find it helpful to separate the light value (how bright the subject is) from the exposure value (what camera settings are used at a given ISO). A quick google search returns a link to Ken Rockwell's site:

https://kenrockwell.com/tech/ev.htm

which describes exactly how I would describe it, unlike the Wikipedia article which seems to use EV for LV and does not explain its connection to the ISO. Good old Ken.

Another good link:

https://www.scantips.com/lights/evchart.html

Thanks for the links, Dem, which confirm what you said. It bothers me too that the Wiki uses EV where it really should use LV.

[ajax]

Sorry it took me a little more than a day to get back and find such a terrific discussion taking place.

My question was intended to be somewhat general but some seem interested in knowing the specific equipment. My camera is a Canon EOS 1300D (also known as a Rebel T6), the lens are APS-C type with the wide one being 18-55mm and the long one being 75-300mm. In that, entry level at the low end of the price curve for DSLR equipment.

Even though I'm lacking in experience, I do understand that camera shake can be problematic. However, prior to this discussion I would have thought that this is something the photographer has to deal with. In that, I didn't expect the camera to try and determine if it was shaking. I think like some of those who've commented I'd rather the camera didn't make assumptions absent any actual data indicating such a need. It sounds like I should stop using auto ISO but must admit I need to investigate whether my camera uses an ISO setting as a maximum or an actual value. I've been assuming it to be actual.

My reference to normal of course is accompanied by an admitted lack of experience/knowledge. My thinking, so far, has been that increasing the ISO is undesirable and should be seen as a necessary evil when conditions warrant. As such, my thinking would be that the camera favors lower ISO which comes at the price of slower shutter speed. The photographer, someday me, should know that the camera selected too slow a shutter speed for the situation and then increase the ISO to allow the camera to raise (i.e, rather than the other way around which is what seems to be happening).

I usually use the wide lens when shooting landscapes, or seascapes in my case, (i.e., scenes expected to be still) and maximizing depth of field is desired. However, every now and then I spot something that looks like it might be better with more magnification which causes me to want the longer lens but also good depth of field. Even at 75mm the camera seem to want shorter shutter speed and increases ISO.

I've sort of realized that magnifying the scene is going to affect the depth of field and my limited experience means I don't have proper understanding of how such shots might turn out. However, even with the long lens there are occasions when maximizing depth of field seems to be desirable. Depending on the situation it might force getting out the tripod. Again isn't this something that the camera should leave to prerogative of the photographer?

One of the things I've been using the long lens for is spontaneous bird shots. In that, even if you can hold the camera steady the subject is apt to move. In some cases, you intentionally pan (i.e., move) the camera. For example, when shooting a flying bird. Therefore, while I intuitively agree with those who prefer choosing all/most settings themselves, this situation is one where you need to be ready. In fact, it created the need for me to buy a second camera body. While I wouldn't want to boast about the quality of pictures, I have had fairly good results shooting at shutter speeds in the 160 - 320 range which requires low aperture settings to keep the ISO from rising. However, while proper focus becomes a problem these shots, seem to me, typically come out better with little depth of field. It is when set this way and I quickly wanted to shoot a landscape that I noticed what happens when switching to aperture priority.

Finally, it has been my understanding that my (Canon) APS-C lens differ from full frame by a factor of 1.6. However, I thought this was a multiplier for the full frame equivalent. In that, my lens set at 300mm is like a full frame that is a bit less than 200mm. Philip (MrB) seems to be saying the opposite. If so that makes camera shake a much bigger problem than I've been thinking. Can anyone refer me to a source of information about this relationship?

[pschlute]

First off aps-c versus FF. The FF sensor is larger and will always show a wider FOV for any given focal length. So a 300mm on your 1.6 crop sensor will give the same FOV as a 480mm lens on a FF sensor.

This crop-factor is a property of the sensor not the lens. A 300mm lens is the same whether it is a aps-c specific lens or a FF specific lens. Although some aps-c lenses will not fully cover the FF sensor.

As far as auto ISO behaviour goes, you have read the comments above. The answer is not to use auto ISO if you don't like the results it gives you.

[Stagecoach]

Sorry it took me a little more than a day to get back and find such a terrific discussion taking place.

My question was intended to be somewhat general but some seem interested in knowing the specific equipment. My camera is a Canon EOS 1300D (also known as a Rebel T6), the lens are APS-C type with the wide one being 18-55mm and the long one being 75-300mm. In that, entry level at the low end of the price curve for DSLR equipment.

David,

The reasoning behind wishing to know the specific camera model you were using is that auto ISO can work in different ways with different manufacturers differing models of cameras. Without this specific information there can be much 'speculation' included within replies that are attempting to fathom out why a specific situation is happening.

Original posting .....................

When using the longer zoom lens with camera set to aperture priority and automatic ISO it seems odd, to me, that the camera chooses what, I'm thinking, are high values for both ISO and shutter speed. The automatically selected values are most extreme with maximum zoom. For example, with aperture set to f22 and lens at 300mm in what I'd call normal daylight the camera chooses ISO 800 and shutter speed 500. Both values seem, to me, to be extreme. Why doesn't the camera opt for lower values for both.

I have recognized when moderating the amount of zoom these values seem to come back into a somewhat more normal range. For example, the same shot as above with lens at 75mm the camera changes to ISO 400 and shutter speed 160. That still seems like pretty fast shutter speed and relatively high ISO.

From your description, and my personal interpretation of it, it 'suggests' that your camera with Auto ISO selected is taking account of the focal length being used when determining the balance between Speed and ISO variation when set to Aperture Priority (fixed aperture).

From what I can gather reading information about your specific camera, this is not the case. What you have described as your findings makes no sense to me at present.

An easy test to confirm what is happening whilst using Auto ISO would be as follows;

Set camera to 'Aperture Priority' with Aperture selected as f/22.
Point the camera at the sky at minimum zoom and note Speed and ISO readings.
Change to maximum zoom and note Speed and ISO readings.
Have they changed?

Edit - Additional Information Found

Delving further into this I have come across this article regarding Canon Auto ISO operation in Aperture Priority https://chrisbrayphotography.com/tips/auto_iso.php which is unclear of what Canon models it refers to but has Canon 'EOS 5D Mk11' written on a diagram. The explanation describes that ISO is determined by Focal Length in this configuration.

[Manfred M]

David - the rule of thumb regarding ISO (and this goes back to the film days too) was to shoot as slow an ISO as is practical to maximize image quality. In other words, the objective is to get an image that is free of unwanted camera movement and using a high enough ISO setting to achieve this is perfectly acceptable

The extra noise, reduced colour bit depth and reduced dynamic range associated with shooting at higher ISO are all considered acceptable trade-offs as camera shake will definitely ruin an image. I can generally live with a bit of digital noise because I can treat that in post-processing and get acceptable results. Anything more than a tiny bit of camera movement results in pictures that just don't work and are something I will delete immediately.

Most serious landscape photographers use a sturdy tripod as that allows them to shoot at low ISO values and still get tack sharp pictures. It's not that they like carrying tripods, but rather that they want to get extremely sharp images shot at the camera's "base" ISO . When it comes to bird photography, I know a couple of outstanding photographers who tell me that they have no issue at all shooting at extremely high ISO values (bird photographers also seem to own the most expensive cameras and lenses).

Modern digital cameras are simply computers that take pictures. Depending on the make and model, they allow the photographer to use the camera much like a point & shoot camera where the camera will make every decision for the photographer other than selecting the focal length of the zoom lens and how the image is framed. On the flip size, the photographer can also constrain what the camera does and will let him or her make their own decisions, right down to doing everything manually including focusing, setting the aperture, ISO and shutter speed. If the photographer knows what they are doing, they will generally get better pictures than one where the camera makes all the decisions for them. I'm not suggesting that you shoot at 100% manual; I generally use aperture priority mode for most of my outdoor work and in certain situations will use shutter priority. I generally do not use auto-ISO. I don't use all the fancy focusing modes that my camera has and virtually always use a single focus point; I focus and then reframe my shot. Indoors, especially in the studio, I shoot 100% manual exposure.

The comments that Philip and Peter have made about the crop factor is 100% accurate. The focal length of the lens is measured in mm and does not change. Crop factor is 100% related to the sensor size. The difference between crop factor lenses and full frame lenses is something called the "image circle", i.e. the size of the sensor that they are designed to cover.

Finally, if you want more information, have a look at the tutorials here on the CiC site. All of the questions you have asked have answers there.

[ajax]

Peter, thank you! While not related to my original concern my misconception of lens crop factor is a case of gross misunderstanding on my part. Very nice explanation that should help me retain this new knowledge.

[ajax]

David,

...

An easy test to confirm what is happening whilst using Auto ISO would be as follows;

Set camera to 'Aperture Priority' with Aperture selected as f/22.
Point the camera at the sky at minimum zoom and note Speed and ISO readings.
Change to maximum zoom and note Speed and ISO readings.
Have they changed?

...

Yes, I did that (see original post) and they do change but my thinking was that the ISO was still set very high. Since I have 2 camera bodies that are the same I also verified that I got the same result with the long lens on both bodies. This implied that it probably was not a camera malfunction that I was dealing with.

[ajax]

In general it sounds like I need to start setting the ISO manually. Then I need to experiment a bit with higher shutter speed and ISO and try and assess the affects on resulting photos. My instinct is to want to maximize quality even though my equipment may be a bit lacking. However, I don't yet appreciate how severe the affect of increasing ISO when compared to ability to hold the camera steady.

I do have a pretty good tripod but to date I'd say my picture taking is opportunistic. In that, I carry the cameras when doing other things and only shoot photos when something interesting is encountered. I'm afraid I haven't yet figured out how to incorporate the tripod. However, the tripod definitely could be incorporated into experiments.

[Stagecoach]

Yes, I did that (see original post) and they do change but my thinking was that the ISO was still set very high. Since I have 2 camera bodies that are the same I also verified that I got the same result with the long lens on both bodies. This implied that it probably was not a camera malfunction that I was dealing with.

Thanks for that confirmation David.

I use Nikons and use their Auto ISO function that takes no account of focal length very regularly.

Having that put into the equation I would find 'unpredictable' as I often use zooms, but maybe it would suit the shooting style of some.

[pschlute]

In general it sounds like I need to start setting the ISO manually. Then I need to experiment a bit with higher shutter speed and ISO and try and assess the affects on resulting photos. My instinct is to want to maximize quality even though my equipment may be a bit lacking. However, I don't yet appreciate how severe the affect of increasing ISO when compared to ability to hold the camera steady.

Your instinct is right in wanting to use as low an ISO as possible. But as Manfred's post above detailed, there is a trade-off and if you don't get the balance right your picture will not be sharp.

ISO in digital cameras is an amplification of the signal. When amplified too much you see increased noise and reduced quality as Manfred explained above. But the important thing about digital ISO (unlike film ISO) and the effects of digital ISO is that being digital it can be manipulated. As technology improves, the bad effects of noise for example, can be dealt with better in post-processing software such as Photoshop so that noise can be reduced without impacting sharpness as much. The technological advances in sensors has also improved since the first digital cameras. Ten years ago I would not want to use a higher ISO than 400 as the noise would be noticeable from 800. Today on a newer camera I can happily shoot at 1600 and get an image with low noise.

So my advice would be to continue your experimentations with finding the right balance of ISO versus shutter speed, but to also invest more time in PP noise reduction techniques.

[MrB]

David, as hinted in Post #9, in your experiments with Aperture Priority Mode and Auto ISO, it might be helpful to you to include the effects of you changing the size of the aperture on your camera's chosen Auto ISO value. If your concern is depth of field you can investigate that by using the calculator here on CiC -
https://www.cambridgeincolour.com/tu...calculator.htm
For example, if you were photographing a bird 10 metres away from you with a 300mm focal length, increasing aperture size to f/11 (instead of f/22) would still give you about half a metre of depth of field, which should be sufficient, while the extra exposure at f/11 should cause your camera to reduce the ISO value to a level at which image noise is not a problem.

Philip