Question on Relation between Manual Focus and Focus Area

[JBW]

A perfect example of the effect of the different options Sony offers.

[Stagecoach]

Only explanation that makes any sense, the exposure of all 5 images is 1/250; f1.8; ISO 400.

There's also quite a colour change between images.

[CatherineA]

There's a possibility the light caused this variation due to frequency.

Can you tell us what sort of light this is please?

Thank you very much Grahame! I am embarrassed that I hadn't thought of that. I had moved the postcard to different places today to try to get a consistent light for the shots but everywhere else seemed worse - even the clouds were moving too quickly to do daylight shots. What I should have done though was take a few shots with each Focus Area setting with this light. That would have shown me exactly what you suspected - that the light was causing the variation.

Well, red cheeks but no longer perplexed.

[Stagecoach]

Thank you very much Grahame! I am embarrassed that I hadn't thought of that. I had moved the postcard to different places today to try to get a consistent light for the shots but everywhere else seemed worse - even the clouds were moving too quickly to do daylight shots. What I should have done though was take a few shots with each Focus Area setting with this light. That would have shown me exactly what you suspected - that the light was causing the variation.

Well, red cheeks but no longer perplexed.

You are welcome Catherine but I can not guarantee what I am suspecting without knowing the type of bulb in that desk lamp.

Hopefully our resident expert on 'light' theory, Ted, will comment on this.

[CatherineA]

You are welcome Catherine but I can not guarantee what I am suspecting without knowing the type of bulb in that desk lamp.

Hopefully our resident expert on 'light' theory, Ted, will comment on this.

It is an "OTT-LITE VisionSaver Plus." I tried to find out more about this light years ago because I used it daily for hours on end for detailed colour work. The company replied that the details that I was looking for were confidential. I hesitated before continuing to use it but it worked so better than anything else I had come across that I stuck with it. For my purposes it performed well and I was unaware of fluctuations in power and colour rendition.

Tomorrow I will experiment a bit more with the camera and its Focus Area choices.

[Manfred M]

It is an "OTT-LITE VisionSaver Plus." I tried to find out more about this light years ago because I used it daily for hours on end for detailed colour work. The company replied that the details that I was looking for were confidential. I hesitated before continuing to use it but it worked so better than anything else I had come across that I stuck with it. For my purposes it performed well and I was unaware of fluctuations in power and colour rendition.

Tomorrow I will experiment a bit more with the camera and its Focus Area choices.

Try shooting at line frequency (1/60th sec) or even multiples of line frequency (1/30th, 1/15th etc.)

[Stagecoach]

Try shooting at line frequency (1/60th sec) or even multiples of line frequency (1/30th, 1/15th etc.)

Would a longer exposure also smooth things out?

[Manfred M]

Would a longer exposure also smooth things out?

Yes - we discussed lights that claim to have a high CRI during my printing glass. The instructor had tested this brand of light (Ott Light) and suggested that that it did not live up to its claims on light quality. So far as I know, they use a CF light source that runs either at line frequency or 2x line frequency, so working at fractions of line frequency should give full multiples of the wavelength, so the light should come out looking consistent in the image.

When Catherine shot at 1/250th sec she would be getting just a part of the wavelength. The phosphors in a fluorescent light stop emitting light as soon as they are not excited and they decay in nanoseconds, so that definitely explains Catherine's results with varying light intensity.

[dje]

Grahame and Manfred

I think you are onto it. I think it is related to fluoro light flicker and a fairly fast shutter speed. I was able to simulate the effect that Catherine is getting using an old fluoro desk lamp.

The actual amount of light captured depends on the timing of the shutter in relation to the AC power cycle. It is basically random and is not related to focus area at all.

Out of interest, I have found that the Brightness Value is a field in the camera EXIF data and is an indication of the amount of light hitting the scene during the exposure, based on the in-camera meter. One of Catherine's Brightness Values is somewhat different to the others but I suspect the metering calculations are also random depending on the timing of the metering in relation to the AC power cycle.

Some good detective work there!

Dave

[rpcrowe]

I am still learning the complexities of my A6500, which I believe has a similar focusing system to the A7iii. I am under the impression that choice of focus area is only valid when using AF. I could be wrong but, I don't think that the focus area has any bearing on manual focusing.

When I do manual focus, I use a combination of Focus Assist (zooming in on the area) and Focus Peaking with the bright areas (depending on the color selected) indicating the area in focus. A neat thing about the touch screen of the A6500 (I am not familiar with other Sony bodies) is that when I am working in manual focus, double touching the area on the LCD Screen which I desire to focus will cause the camera to zoom in on that area for focus assist. It will NOT focus on the area which has been double clicked but, this will provide focus assist on that area. This could "ALMOST" be considered semi-manual focusing...

This is a lot better than manual focusing with a DSLR. DSLR cameras IMO are generally lousy at manual focusing unless you are able to replace the viewfinder screen. Many of the older manual focus cameras had either a split image rangefinder or a center focus point built in. It was a LOT easier for me to focus with an older non-EOS SLR than with a later DSLR which was intended for auto focusing.

The little Sony A6500 is a delight to focus manually. However, best of all is the Eye AF which is the best thing since sliced bread! Unfortunately CaNikon neglected to or decided against equipping their mirrorless offerings with Eye AF. I will admit that the Face Detect in Live View of my Canon 6D Mark-2 hits focus on the eyes pretty regularly. But, you don't actually know if the correct focus has been achieved until you view the image you have shot. That strikes me as a shame because, unless you have used Eye AF, you probably don't have any idea how nice it is. No more shifting the AF point to go over an eye or, even worse, focusing and recomposing which can be a bearcat if you or your subject moves just a little when shooting with a very narrow DOF......

[Stagecoach]

Grahame and Manfred

I think you are onto it. I think it is related to fluoro light flicker and a fairly fast shutter speed. I was able to simulate the effect that Catherine is getting using an old fluoro desk lamp.

The actual amount of light captured depends on the timing of the shutter in relation to the AC power cycle.

Some good detective work there!

Dave

For info, I also undertook some test shots today in a dark room with the only light source being an 'energy saving flouro'.

6 Shots at 1/250th sec each had a slightly different exposure and hint of colour difference.
6 shots at 1/20th sec each shot identical exposure and no colour difference visible.

[Manfred M]

Dave / Grahame - I did a similar experiment a number of years back and got similar results to what you did, so I'm not surprised.

In the old days of CRT video displays there were somewhat related issues when shooting computer screens or television sets because the trace lines would show up in the shots. Cheaper video cameras could not cope, but higher end ones had variable shutter speeds and the camera could be set up to eliminate the flicker.

[dje]

This lighting issue depends on the type of fluoro and in particular the type of ballast. With the older magnetic choke type ballasts, the tube is fed with the AC line frequency (50Hz or 60HZ). With the latest electronic ballasts, the AC line frequency is converted into a high frequency signal that feeds the tube. With a frequency of say 20KHz, flicker is non-existent.

Dave

[CatherineA]

I took seven shots of the same postcard today, each at ISO 100, f/22 and 2s. The exposure was 0 for each photo, as it was for all of the images in the previous set, however this time there was not as much variation in Brightness Level (the range was 1.64 - 1.77 and the levels were clearly unrelated to the Focus Area).

So, thanks to all of you for clearing up my misunderstanding. I see now that it was the fluctuations in the light frequency were responsible for the differences that were troubling me!