What am I doing wrong?? (DOF question)

[Chuck Lee]

I'm going to guess 5DMII with 24-105 f4L.


Based on the info below I'm also going to guess that you were 3ft-5ft away and shooting at 70mm - 105mm to get this close up head shot.


Min focus distance is aprox 18". I'm thinking that if your in a hall you can get a little further away. Say 4-6 ft.


@ 50mm- DOF @f8 from 4ft= 11" 6ft = 25.5"


@ 70mm - DOF @f8 from 4ft= 5.4" 6ft = 12.5"


@ 105mm- DOF @f8 from 4ft = 2.25" 6ft = 5.4"


Like Sean said: What were the settings?


The catch 22: The further back you get the more DOF you will have. The more you zoom in the less DOF you have.


Try shooting at a wider angle and cropping or shoot at a higher aperture. If you can stand another stop go to f16 same composure/crop.


Sorry for stating the obvious.


I'm still struggling with FF and this very same thing.


Oh and...........Don't lick the monkey! It'll leave a bad taste in your mouth I'm sure.


Chuck

[Daniel Browning]

Good post Chuck Lee, just one minor correction:

Try shooting at a wider angle and cropping...

Cropping in post ("digital zoom") will make the DOF just as thin as if you had zoomed optically. The only time a wider angle has more DOF is if you don't compensate for the change in framing (composition). When you compensate for it by moving closer or cropping, it makes the DOF thinner again.

[Madison]

Flash is not allowed anywhere. They're very strict.


It's an old antique building. The stairwell is a huge square room. The head is in one corner. But the light is tricky. I will reshoot in a few days with all these wonderful tips (thanks everyone: i am most grateful for it) and will come back to discuss further!

[Chuck Lee]

OK, hang on Daniel,


Your confusing me........ again. LOL


According to the DOF calculator my distance of acceptable sharpness increases with:


1) increased aperture (i.e. larger number = smaller iris)


2) increased camera to subject distance


Let's say the subject is yardstick laying on the ground. My camera is on a rail 45 degrees to the ground plane. I am focusing on the 18" mark with center focus point.


At10ft awayI take a photo ofthe subject using 100mm at f4, I should get the same distance of acceptable sharpness as if I had taken the same photo from 5ft away with a 50mm at f4. Themaindifferencesare the field of view and the amount ofbackground blur. This distance on a 5D FF is .71 ft for both shots. So I see the middle 8-10" of the yardstick in acceptable focus.


So what happens when I takea photo from 10ft away with a 50mm at f4?The distance(or depth) of acceptable sharpness is now 2.97ft. I see almost all of the yardstick in acceptablefocus, do I not? The difference is that the yardstick appears half the size in the frame as the other two images.


That's easy to remedy. I just crop the image in half. It goes from 3000 pixels to 1500 pixels. The yardstick now appears to be correctly porportioned in the frame.


The yardstick also appears to be almost completely in focus unlike the other two shots.


What an I missing Daniel? and I mean that respectfully and sincerely.......


Chuck

[Daniel Browning]

At 10ft away I take a photo of the subject using 100mm at f4, I should get the same distance of acceptable sharpness as if I had taken the same photo from 5ft away with a 50mm at f4.

Agreed.

The main differences are the field of view and the amount of background blur.

I concur. For the reader I would add that perspective is different and point out that it's only the angular field of view (angle of view) that is different: the absolute field of view (i.e. subject framing in feet) remains the same.

This distance on a 5D FF is .71 ft for both shots. So I see the middle 8-10" of the yardstick in acceptable focus.

Yep. Your DOF calculator uses 0.03mm for an acceptably sharp CoC on the 5D2. I get the exact same numbers as you. Here they are in metric:

50mm 1.524m distance f/4 DOF = 0.21m
100mm 3.048m distance f/4 DOF = 0.21m

So what happens when I take a photo from 10ft away with a 50mm at f4? The distance (or depth) of acceptable sharpness is now 2.97ft. I see almost all of the yardstick in acceptable focus, do I not?

Yes. That's what I get too:

50mm 3.048m distance f/4 DOF = 0.90m

The difference is that the yardstick appears half the size in the frame as the other two images. That's easy to remedy. I just crop the image in half. It goes from 3000 pixels to 1500 pixels. The yardstick now appears to be correctly porportioned in the frame.

So far so good.

The yardstick also appears to be almost completely in focus unlike the other two shots.

This part is incorrect. It will have thin DOF like the other two shots. (In some cases it will be blurry *all over* the image, but that doesn't count as a deeper DOF advantage because you could add blur to the thin DOF shots too if you wanted to go for that blurry look.)





I think if you actually try the experiment you described you'll see what I'm talking about. I think I recall seeing similar experiments already on the web, so we should be able to find one with GIYF.

The reason why cropping affects DOF is the same reason why print size affects DOF: reproduction magnification. If you crop the image in half *and* display it at half size (print or screen), then the reproduction magnification stays the same and therefore the DOF is the same. But if you crop the image in half and display it at the *same* size, (as people do in real life) then the reproduction magnification increases, and the DOF goes back to beeing thin.

Cropping in post is very similar to using a "crop" sensor, and has the same result in DOF (if the pixel size is the same on the crop camera and the FF camera). Let's take the 30D and 5D2 for example, since they have the exact same pixel size.

Using 3m focus distance and h/CoC=1200:

30D 50mm f/4 DOF = 0.54m
5D2 50mm f/4 DOF = 0.88m

With the same lens, the 30D has thinner DOF, because it is only seeing the center crop of what the 5D2 sees, but it is still magnifying it to the same print size. What if you crop the 5D2 digitally so it matches the framing of the 30D?

30D 50mm f/4 12x18 print DOF = 0.54m
5D2 50mm f/4 digitally cropped by 1.6X, 12x18 print DOF = 0.54m

Now the 5D2 has the same thin DOF as the 30D. Another variable is print size. If you print it at 1.6X smaller size (7.5x11.25) the DOF goes back to being the same. In fact, it looks like you took a knife to the 5D2 print and just remove the center portion to make the 30D print:

30D 50mm f/4 7.5x11.25 print DOF = 0.88m
5D2 50mm f/4 12x18 print DOF = 0.88m

In other words:

1. 5D2 + lens A + cropping = thinner DOF than 5D2 + lens A + no cropping
2. 30D + lens A = thinner DOF than 5D2 + lens A + no cropping

The best way to look at it is the *total* magnification from the size of the original subject to its size to the viewer.

Any thing you do to make the subject bigger for the viewer will make DOF thinner. Get closer, use a longer lens, use a smaller sensor (with same lens), crop digitally, print larger, move the viewer closer to the print, etc. All of that increases the magnification, and any increase in magnification makes the DOF thinner.

[wickerprints]

Put a bit more simply, the CoC is partly a function of reproduction magnification and partly a function of viewing distance. Changing this magnification necessarily changes the CoC, and in turn, DOF, since now you have affected what you consider to be "acceptably sharp."


DOF is a convenient concept but it is not really how sharpness as a function of focusing distance really behaves. The loss of sharpness is not sudden but a continuous gradation.

[Chuck Lee]

Daniel,


Truly, you have a dizzying intellect!!


All righty then. It's yardsticks at dawn!!


LOL!!


I'll post results tomorrow.


ITMT...............


Madison,


Do you have anyone who can help you? If not, I'd be willing to bet that you could finda museum employee to hold a reflector for you for five minutes.

[wickerprints]

1. Check your AF accuracy. You may need a microfocus adjustment.
2. You can slightly reduce camera shake further with a makeshift "monopod." Here's how it works: (a) Take a bolt that threads into the tripod mount of your camera. (b) Measure a length of string that is longer than the distance from the floor to eye level. (c) Tie one end of the string to the bolt. (d) Get a flat washer and tie it to the other end of the string. (e) To use this device, screw the bolt into the tripod mount, unwind the string, and let the washer fall to the floor. Step on the washer and pull the string taut as you compose your photo. The induced tension on the string eliminates vertical camera shake. (f) It also helps to use the timer feature.
   
3. Increase ISO by one more stop.
4. Understand the shape of your subject and select a focus point that has the appropriate contrast and places the DOF in such a way as to maximize the amount of subject in acceptable focus. That is to say, if AF selects a point on the subject that is closest to you, then you are only utilizing the rear DOF. If the AF selects a point on the subject furthest away to you, then you are only utilizing the front DOF.

[Chuck Lee]

wickerprints,


Have you actually tried #2 with success?


Chuck

[David Selby]

your shutter speed is far too low for a hand held image, and your minimum focus distance my be pushing it on that camera.





Finally, you may not have enough light for accurate focusing, but certainly your shutter speed is way to low for 58mm it needs to be at least 1/focal length for hand holding with a full frame and 1/focal length x 1.6 for crops. I recommend always getting even higher if possible.