Hyperfocal and Zone Focusing with Adapted Lenses

[dbmiller]

I have been trying to wrap my mind around it, and I think there's a problem using adapted lenses.

The scale on an old lens is probably meant for film at the original "Circle of Confusion" size (.03mm).

In the old days, I would line up the infinity mark on the lens with the f-stop I have chosen, and I have the maximum (hyperfocal) DoF, right?

But with the adapted lens on the NEX, the hyperfocal distance is not the same, because the CoC is different (.03m / 1.5 = .02mm).

Taking a real world example: 50mm lens, at f11. For film, the hyperfocal distance is 24.3 feet. For the NEX, it's actually 36.4 feet. So if I'm focused at 24.3 feet, I won't have enough DoF to reach infinity.

Similarly, if I am at f8, and I look at the scale on my lens, it tells me I can get a DoF ranging from 5 to 7 feet if I focus at just under 6ft. But that's film. In actuality, on the NEX, I lose about a third of a foot (4 inches) at either end.

So trusting the scale on an adapted lens doesn't work! Right?

 

[dbmiller]

I just used Online Depth of Field Calculator to double check my #'s.

For the 5-7 foot setup, use 70 inches as the focus distance, 50mm lens @ f8. For film, DoF is 60" to 83.9". For NEX, it's 63" to 78.7"

Hyperfocal matched my spreadsheet calculations - 24.3' for film, 36.4' on the NEX when using f11

 

[Bugleone]

Interesting,....

Actually, I strongly urge you to do your own test to determine adequate depth of field,...it's not as difficult to do as one might suppose, however, I have found that there is as much 'art' in it as science.

I'm not sure about your figures that suggest a smaller CoC for a digital sensor over film,...but even if you are correct, there is still much to be gained by running your own test according to the size of print to are expecting to make.

In my own use of NEX 3 I always aim at a 20x16 inch print ( and view from 2.5 feet), and in using my Canon FD lenses I give myself a 'safety margin' of one stop. However, in looking at the 20x16 prints I notidce that things change according to both subject and iso used,..also, that the one stop margin is not really vital to the overall quality of the print.

 

[storyteller]

I think he's right. The smaller sensor has a smaller CoC and therefore smaller depth of field at the same distance as a full-frame/sensor with the same focal length. To get the same framing/shot as the full-frame film/sensor you have to move the NEX farther back which increases the depth of field at a greater rate than the smaller sensor decreases it. The end result is that a smaller sensor perceptually has a deeper depth of field but the DoF markings on manual lenses will be off because they will (counter intuitively) be narrower.

Thanks, dbmiller, I'd researched the math/science here but never really processed how it affected hyperfocal distances on the NEX system. I feel dumb for not thinking it through as you did but I think you just explained why several shots I took didn't work.

 

[dbmiller]

Quick Test Result

I've been busy, but tried to throw together a quick test. I tested my Canon 50mm LTM lens mounted via an LTM->M adapter, plus the Hawk's M->NEX adapter. I set the lens to f5.6, which according to the scale on the lens, would provide a DoF from nearly 6ft to 8ft.

I then arranged some dominoes on my pool table at the following distances...

• 5/11 at 5'11"
• 6/0 at 6'0"
• 6/1 at 6'1"
• 6/2 at 6'2"
• 6/3 at 6'4"
• 7/0 at 7'0"
• 6/8 at 7'8"
• 6/9 at 6'9"
• 6/10 at 6'10"
• 6/11 at 6'11"
• 8/0 at 8'0"

I then took an image with the LTM lens, as well as my 18-55 kit zoom set to 50mm. For the kit zoom, I focused on the 7/0 domino, which I believe is the focus point from the scale.

Attached are two 100% crops from each lens. They are nearly identical, although the kit lens might be focused a little further back than the Canon LTM. Both appear to have the same DoF. My opinion is that the domino at 6'3" is acceptable for the front, and 7'9" for the back. Which is a loss of about 3" from what the scale on the lens says would be in focus for a FF camera/lens. The total DoF (18 inches) is still slightly more than DofMaster computes for this situation (15.6 inches), but that may depend on what you find acceptable. But it is definitely less than the scale on the lens.

 

[davect01]

As much as I appreciate focal distance measuring, the manual focusing and magnifier built into the NEX work very well.

 

[dbmiller]

As much as I appreciate focal distance measuring, the manual focusing and magnifier built into the NEX work very well.

Understood. Most of the time, the NEX works well enough to operate it WYSIWYG mode. But I first noticed this issue when I was shooting in a low-light, moving condition where it was difficult to get the focus right using the LCD, and didn't have time for fine tuning. So I turned to zone focusing, expecting it to work, and scratched my head for a bit when the images didn't come out as I expected.

 

[Bugleone]

As much as I appreciate focal distance measuring, the manual focusing and magnifier built into the NEX work very well.

That's not really whats concerned with 'depth of field'. The actual point of focus can be as sharp as can be but the rest of the 'area of sharpness that is acceptable' depends largely on the size of print, actually,...and that determines apertures.

 

[storyteller]

Thanks for this quick test.

 

[davect01]

Understood. Most of the time, the NEX works well enough to operate it WYSIWYG mode. But I first noticed this issue when I was shooting in a low-light, moving condition where it was difficult to get the focus right using the LCD, and didn't have time for fine tuning. So I turned to zone focusing, expecting it to work, and scratched my head for a bit when the images didn't come out as I expected.

That's not really whats concerned with 'depth of field'. The actual point of focus can be as sharp as can be but the rest of the 'area of sharpness that is acceptable' depends largely on the size of print, actually,...and that determines apertures.

I totally get your frustration and understand the concept. The kinds of shots you are taking require a bit of patience and improperly operating equipment just makes things more frustrating.

The adapters that I have with my manual lenses does throw off any proper measurements. On the flip side when focusing to infinity there is a tenancy to go out beyond and the whole image is out of focus. There are a few tricks to remedy this, but I wish it worked properly from the get go.

I guess what I was trying to say is that 99 percent of the non photographic people you will show your work to won't care about focal lengths, depth of field etc. In fact you start mentioning these things and they get a blank stare and quickly loose focus.

 

[dbmiller]

An old topic revisited

Revisiting this thread from long ago due to a recent thread.

I found a reference on Wiki that describes the effect of the crop factor on the circle of confusion and depth of field:

Adjusting the circle of confusion diameter for a lens’s DoF scale

The f-number determined from a lens DoF scale can be adjusted to reflect a CoC different from the one on which the DoF scale is based. It is shown in the Depth of field article that

DoF = (2 N c ( m + 1 )) / (m^2 - ( ( (N c) / f ) ^2 ) )

​

where N is the lens f-number, c is the CoC, m is the magnification, and f is the lens focal length. Because the f-number and CoC occur only as the product Nc, an increase in one is equivalent to a corresponding decrease in the other, and vice versa. For example, if it is known that a lens DoF scale is based on a CoC of 0.035 mm, and the actual conditions require a CoC of 0.025 mm, the CoC must be decreased by a factor of 0.035 / 0.025 = 1.4; this can be accomplished by increasing the f-number determined from the DoF scale by the same factor, or about 1 stop, so the lens can simply be closed down 1 stop from the value indicated on the scale.
​

Since the crop factor of the NEX is 1.5, stopping down an extra stop is just about enough to adjust the DoF to match the scale on a legacy lens.