Aperture priority and focal length

[cauger61]

I understand that "Aperture-priority" mode is good for "bokeh" effect and also for controlling the depth of field. In A-mode, it is suggested that in order to have a shallow depth of field, one needs to set the maximum aperture. OK that be taken, my confusion arises, as to what focal length should one use? because, in zoom lenses, aperture narrows down to as one zooms in. For example, in case of 18-55 lens, f varies from 3.5 to 5.6 and in case of 70-300 lens f ranges from 4.5 to 6.3. Therefore, if one zooms, the aperture value does not remains under the photographer's control. My point of confusion is, in order to use the maximum aperture, should one stick to wide angle part of the focal length of the lens or what?

[pschlute]

Not all zoom lenses are variable aperture, but as yours are, then as you zoom in the maximum aperture you can use will be narrower.

But aperture is not the only factor affecting depth of field. Camera to subject distance will also have an effect, so the closer you are to your subject the less the DOF will be.

Focal length can also appear to narrow DOF when using a longer FL. this is largely due to perspective change and magnification rather than actually a property of a longer lens to reduce DOF.

So in short widest aperture/shortest camera to subject distance will give you the narrowest DOF.

Have a look here for a more detailed explanation https://www.cambridgeincolour.com/tu...h-of-field.htm

[cauger61]

Thnx Peter. I am 90 percent clarified. I will further appreciate, if you just comment on my last question — "in order to use the maximum aperture, should one stick to wide angle part of the focal length of the lens".

[DanK]

I think you are confusing two things. The aperture you set affects depth of field. The mode you set, like AV, does not directly affect depth of field. It is just one way to get the aperture setting you need for the depth of field you want.

I agree with Peter: I would start with the tutorial on this site that he links.

As for focal length: this is something that is often misunderstood, but background blur is not the same thing as depth of field. Unfortunately, an excellent web page that showed this with photos has been taken down, but I'll write a brief description. As the tutorial says:

Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length!

So, what happens if you switch from a wide angle to a telephoto and step back so the subject occupies the same portion of the frame? Depth of field will stay the same. Nonetheless, if the background is not very close to the subject, the background will be more blurred with the longer focal length. That is because the longer focal length lens has a narrower angle of view, and its therefore capturing a smaller amount of the background and spreading it more to cover the same framing.

In many cases, the visual appearance depends on both DOF and background blur, so if one wants a very blurred background, one is better off with a longer lens. This is one reason some people avoid short-focal-length macro lenses. Most macro lenses reach the same magnification, so the subject is the same size regardless of the lens, but the background will be more blurred with the longer lens.

I think you should read the tutorial first; that might answer your last question to Peter.

[dem]

Thnx Peter. I am 90 percent clarified. I will further appreciate, if you just comment on my last question — "in order to use the maximum aperture, should one stick to wide angle part of the focal length of the lens".

Yes. For a 18-55 f/3.5-f/5.6 lens you get f/3.5 at 18mm and f/5.6 at 55mm. If you want to shoot at f/3.5, you need to stay close to the 18mm end.

[Manfred M]

Sanjib - the depth of field and bokeh are not related at all to using the aperture priority mode of your camera. They are related to other factors, such as the aperture you use (and that will be in any of the shooting modes; aperture priority, shutter priority, program mode and manual mode), distance from the subject (i.e. amount of magnification) and sensor size.

In terms of the lenses you are using, the maximum aperture does vary. Aperture (f-stop) is just the ratio of the focal length one is using divided by the diameter of the entrance pupil of your lens. Most of the lenses I use are constant aperture lenses, which means that the maximum f-stop is constant throughout the focal range. Lenses that have this feature tend to have both a very limited focal length range AND tend to be expensive.

When it comes to bokeh, this is even more complicated as it refers to the look of out of focus areas in your image, so once again, the aperture used (usually something close to being wide open) and distance to the subject PLUS design characteristics of the lens itself. Some lenses will feature beautiful bokeh and other lenses will produce less pleasing results (hint - lens cost is often the determining factor here too). The distance to the out of focus objects and the nature of the objects also plays a role - distance lights are the part of the image that shows of the bokeh well.

In practical terms, given that you appear to be shooting consumer grade lenses on a crop-sensor camera, I suspect that there will be limits to what you can achieve with respect to both shallow depth of field and bokeh. You will likely find that shooting at or near the maximum focal length of your 70-300mm lens will likely be where you see these effects the most. With your 18-55mm lens, even though the maximum aperture is quite wide, your depth of field will be relatively deep and much of the image will tend to be "in-focus".

Try experimenting with the Depth of Field calculator on this site: https://www.cambridgeincolour.com/tu...calculator.htm

[cauger61]

Thnx a lot Dan for your helpful answer. I have really enjoyed your photos. Therefore, may you get displeased, if I request you just to post the focal length, f-no and the ISO values of some of your images.

With best wishes.

[cauger61]

No, taking everything for granted what all of you are saying, I don't know whether posting this will create your irritation or not, my confusion was -- what focal length one should use in case of creating a portrait with a blurry background. Should it be 18-55mm at 33mm with f/3.5 (33mm gives apox. 50mm on a 1.5x crop factor camera) or it is better advisable to use a 70mm lens at f/4.5?

[cauger61]

The last part of Manfred's post appears to me very straight forward -- "In practical terms, given that you appear to be shooting consumer grade lenses on a crop-sensor camera, I suspect that there will be limits to what you can achieve with respect to both shallow depth of field and bokeh. You will likely find that shooting at or near the maximum focal length of your 70-300mm lens will likely be where you see these effects the most. With your 18-55mm lens, even though the maximum aperture is quite wide, your depth of field will be relatively deep and much of the image will tend to be "in-focus". And, I express my sincere gratitude to him for this very useful piece of advice.

[cauger61]

In practical terms, given that you appear to be shooting consumer grade lenses on a crop-sensor camera, I suspect that there will be limits to what you can achieve with respect to both shallow depth of field and bokeh. You will likely find that shooting at or near the maximum focal length of your 70-300mm lens will likely be where you see these effects the most. With your 18-55mm lens, even though the maximum aperture is quite wide, your depth of field will be relatively deep and much of the image will tend to be "in-focus".

Thnx a lot Manfred!! The last part of your answer appears to be very useful and straight forward to me. As advised, I shall go through the tutorial on this site and shall get back, if I stuck at any point.

I have really enjoyed to see your images entitled "Young bear resting on a log" and "Closeup of a young female grizzly eating sweat sage grass near the banks of the estuary". I would love to know their focal length, f-no and ISO etc. if you don't have any reservation.

With best regards.

[rpcrowe]

I think that this image might clarify some of the above statements...



Although this image was not shot with a particularly wide aperture (f/5.6) it was shot with a long focal length (165mm on a crop format camera, with the dog fairly close to the camera and the background a distance to the rear.

This combination of long focal length and subject placement results in a fairly narrow depth of field with the dog in focus and the background totally blurred.

The "quality" or smoothness of the blurred background is known as "bokeh". In this case, it is very smooth and creamy looking with nothing to take your attention away from the main subject, the dog.

Bokeh or "look" of the out of focus area is basically dependent on two factors:

1. The lens itself. In this case my 70-200mm f/4L IS lens has rounded aperture blades resulting in a smooth and creamy out of focus look...

2. The background included in the image. In this case there is nothing that would interfere with attention placed on the subject. However, often a blurred background is chosen because it separates the subject and prevents the background from attracting attention away from the subject...

So Depth of Field is he amount of the image in focus and out of focus and is quantifiable. While Bokeh is the quality or look of the out of focus areas and is subjective.

[Manfred M]

I have really enjoyed to see your images entitled "Young bear resting on a log" and "Closeup of a young female grizzly eating sweat sage grass near the banks of the estuary". I would love to know their focal length, f-no and ISO etc. if you don't have any reservation.

I have no issue looking up this information, but frankly, I suspect that it will be less useful to you than you think. These two images are cropped so that adds another dimension to the variables. Some of the choices I made when I shot them related to the shooting conditions; I was in a Zodiac (inflatable rubber boat) and leaning on the inflated gunwhale, so I had to be fairly conservative to compensate for the boat bobbing up and down in the water. The area I was shooting was quite overcast, so again, limited lighting.

Both shots were taken with a crop-frame (1.5x) Nikon D90 using the Nikkor f/4.5 - 5.6 80-400mm lens. ISO 2000, focal length 400mm and aperture f/5.6. The Young Bear Resting on Log was shot at 1/1000th second and the Young Female Grizzly was shot at 1/640th second. In other words, I was at the maximum aperture of the lens and it was at its farthest zoom position.

The reason that you have fairly shallow depth of field is the extreme level of magnification I was using in those shots. The characteristics of the types of shots one takes with these types of lenses lends itself to shallow DoF in the images.

[cauger61]

Greatly illuminated!! Thnx a lot.
With best regards.

[William W]

. . . my confusion was -- what focal length one should use in case of creating a portrait with a blurry background. Should it be 18-55mm at 33mm with f/3.5 (33mm gives apox. 50mm on a 1.5x crop factor camera) or it is better advisable to use a 70mm lens at f/4.5?

I advise that you use either 33mm or 50mm or 70mm at the largest aperture you have available ... BUT for ‘a rule of thumb’ to get a blurry background concentrate much more on the ratio of:

DISTANCE from Camera the Subject
to the
DISTANCE from Subject to Background

In simple terms, to increase the ‘blurriness' of “Background Blur”, a major constituent is to have the camera relatively closer to the Subject than the Subject is to the Background.

Try a ratio 1:4. As a starting point, more if you can do it.

e.g. Camera to Subject 10ft, Subject to Background 40ft

WW

[cauger61]

Thnx a lot William!! I have tried with f/4.5 at 40mm and also with f/5.6 at 85mm; with your 1:4 ratio and obtained nicely blurry background portraits. However, one problem that I had to face for those indoor portraits was high ISO that I had to set for keeping shutter speed not below 1/30 for hand-held shots, with VR on. This happened inspite of having daylight, though diffused indoors. I attempted to shot with flash to keep the ISO upto 800, but that over-exposed the shots, with washed away highlights.

[Manfred M]

Thnx a lot William!! I have tried with f/4.5 at 40mm and also with f/5.6 at 85mm; with your 1:4 ratio and obtained nicely blurry background portraits. However, one problem that I had to face for those indoor portraits was high ISO that I had to set for keeping shutter speed not below 1/30 for hand-held shots, with VR on. This happened inspite of having daylight, though diffused indoors. I attempted to shot with flash to keep the ISO upto 800, but that over-exposed the shots, with washed away highlights.

It would be useful if you posted the shots for people to see and comment on.

[pschlute]

I attempted to shot with flash to keep the ISO upto 800, but that over-exposed the shots, with washed away highlights.

Using flash should enable you to use up to your sync shutter speed and your lowest ISO. If your camera/flash is capable of TTL or Pre-TTL you should be able to use an auto exposure program. If it is a manual flash can you not lower the output ?

[William W]

Thnx a lot William!! I have tried with f/4.5 at 40mm and also with f/5.6 at 85mm; with your 1:4 ratio and obtained nicely blurry background portraits.

Good.

. . . However, one problem that I had to face for those indoor portraits was high ISO that I had to set for keeping shutter speed not below 1/30 for hand-held shots, with VR on. This happened inspite of having daylight, though diffused indoors. I attempted to shot with flash to keep the ISO up to 800, but that over-exposed the shots, with washed away highlights.

You have a few considerations/issues here. As Manfred suggested an example UNCROPPED image would assist. With that image you should provide the Exposure data (Shutter Speed, ISO and Aperture) and also the details of the Camera Model; Lens and Flash unit that you used.

***

As general comments, some of the matters that you need to consider are:

1. If you are shooting in Available Light (no Flash), then a Shutter Speed of 1/30s is dangerously slow for Portraiture, no matter if you are Hand Held or using a Tripod. Additionally VR is of little or no help in this situation. The fact is Subjects move, even if you think that they are standing or sitting still and that movement results in Subject Movement Blur and appears as a soft image. For Available Light Portraiture for non-professional Subjects a Shutter speed of 1/125s or faster, is usually safe for healthy Adults, for the feeble, elderly and children 1/250s or 1/400s is usually safe.

2. If you use Flash as the Main Light (Key Light) for Portraits, then the ISO can usually be quite low, depending upon your Camera ISO200~400 would usually be suitable. This range of ISO will typically allow a large range of Apertures from which you can choose, hence allowing you good control of the Depth of Field for the shot.

3. (In simple terms) Flash units can be Manual or Automatic (or both). An Automatic Flash Unit communicates to the camera and allows the camera’s Light Meter to define the amount of Flash that is used. The result of this computation is dependent upon a variety of selections that you make, including, (but not limited to):
a) Camera Mode (Manual, Aperture Priority, Shutter Priority, Program, Auto etc) and
b) Flash Exposure Compensation (FEC)

If the Flash is a Manual Flash, then it might have a power selection, and it might not have a power selection. If it has a power selection, then you can adjust the Flash “power” (a technicality - the term “power” is used loosely, it is not really power that you are adjusting but that is the commonly used phrase).

If the Flash unit is (really) a “Manual” Flash, then there will be no power adjustment and the Flash “power” is controlled by the Flash to Subject Distance.

5. Flash Exposure is a very short duration of light (except for High Speed Sync, which I chose not to discuss here). Being a very short exposure of light, the Flash has the effect of “freezing” mostly all Human Subject Movement, usually resulting in a very sharp and clear image (i.e. no Subject Motion Blur).

6. When you are using Flash and there is also another light source, for example Daylight inside a room, then, in simple terms, you have two Lighting options:
a) the Flash can be used as the Fill Light (The Flash is used to “fill in” the shadows, usually on the face)
b) the Flash can be used as the Key Light (The Flash is the Main Light on the Subject, and the other light is used to light the surrounds, for example the interior and rear of the room)

In both situations (6a and 6b) you need to consider TWO SETS of technical details regarding the exposure. One is the "Flash Exposure" and the other is the "Ambient Exposure".

In simple terms, a typical procedure is:
a) ISO is chosen
b) the APERTURE is chosen and as well as determining the Depth of Field may also have an influence on the Flash “Power”, hence may influence the “Flash Exposure”.
c) the Shutter Speed is chosen and this will directly influence the Ambient Exposure.

It is not necessarily the case that the Camera's Flash Sync Speed is chosen for the Shutter Speed. In many situations a slower Shutter Speed than the Flash Sync is chosen and this technique is called “Dragging the Shutter”. This technique is used to make appealing blend of the "Flash Exposure" as the Key Light on the Subject and the "Ambient Exposure" whcih is lighting the room.

The above is a very short overview of some of the main technical considerations involved in Portraiture Photography.

I advise that you take this a step at a time. It is not rocket science and if one works logically each step will fall into place. The first step is for you to post and example UNCROPPED image with some technical details and then take it from there to make some simple Portraits using the Flash as the Key Light.

WW

[cauger61]

OK!! Thnx a lot. I am going to post some hand-held shots with their exif data, for people to comment upon. With best regards.

[William W]

OK!! Thnx a lot. I am going to post some hand-held shots with their exif data, for people to comment upon. With best regards.

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