LF depth of field question

[Augenblick]

Hello. I am new to LF photography. Recently I purchased a superspeed graphic with a 135mm Rodenstock lens. I understand that 135mm focal length in large format is approximately equal to 40mm focal length in 35mm format. Does this mean that the Rodenstock, when fitted to the Graphic, has a dof equivalent to that of a 40mm lens in 35mm film format, or does it mean it has a dof of 135mm lens in 35mm film format?

What would the equivalent dof table for my lens be? I am referrring here to a 'simple' dof table, keeping the film plane parallel to lens, i.e no tilts or swings or drops which would alter the dof.

 

[pau3]

Depth of field depends on the focal length and on the film format.
Given two equivalent focal lengths (135mm with 8X10 and 40mm with 35mm)
the larger the film format the thinner the depth of field. By this reason, it
is difficult to have small depth of field on small digital sensors.

You can find a small depth of field calculator here.

 

[blackwave]

The way I have understood it (and correct me if I'm wrong!) is that DOF depends on focal length only. So the DOF on a 135mm lens in 35mm is the same as 135mm on LF, though the perspective is different because of the format. This explains why a "normal" lens on 35mm has far more DOF than a "normal" on LF.

Clear as mud?? 🙂

 

[rogue_designer]

It gets somewhat complicated though...the apparent depth of field changes due to the relative size of the circle on confusion in relation to the enlargement.

So even though the circle of confusion is the same size from a 135mm lens on the negatives, it must be magnified many times over (7-9x) to get an 8x10 print from 35mm, but from a LF negative, only once or twice. This has a significant effect on the apparent depth of field.

 

[blackwave]

It gets somewhat complicated though...the apparent depth of field changes due to the relative size of the circle on confusion in relation to the enlargement.

So even though the circle of confusion is the same size from a 135mm lens on the negatives, it must be magnified many times over (7-9x) to get an 8x10 print from 35mm, but from a LF negative, only once or twice. This has a significant effect on the apparent depth of field.

This suggests that print size affects apparent depth of field. Is that correct? So a 4x5 contact would have more apparent DOF than the same negative enlarged to 30x40"? I guess I'm not understanding how this would affect apparent DOF and not the entire image. Oh, down the rabbit hole we go! 😉

For full disclosure, I don't fully grasp the circle of confusion concept, because, well, I haven't needed to!

 

[rogue_designer]

that's correct because what your eye would perceive as sharp enough to be considered "in focus" in a contact print, when enlarged would become obvious that it was not really part of what was in focus, the apparent depth of field changed to a narrower field as the larger circles of confusion at the outer edges of the focus area were enlarged.

Don't know if that makes sense or not.

 

[David Goldfarb]

DOF depends on three factors, the focal length, the value for acceptable circle of confusion for the format (which makes certain assumptions about print size, viewing distance, and the limits of human vision), and the subject distance. The value for acceptable circle of confusion is generally larger for larger formats, because it is assumed that the print will not be enlarged as much, but this allowance for a larger circle of confusion doesn't keep pace with the amount of blur that occurs at a specified distance from the focal plane, so as you move up in format, you have less DOF than you would at a smaller format, given the same composition and the use of lenses of analogous focal length.

On the other hand, the out of focus areas will be softer and the sharp areas of the image will be sharper with a larger format in general, and with large format, you have the option of adjusting the plane of focus, so there are other things to consider.

 

[Al Kaplan]

While it's true that you can adjust the plane of focus, the coverage of the lens restricts you when using front swings and tilts. Adjusting the back swings and tilts changes the perspective, such as making buildings look like they're not falling over backwards. You can always use both back and front movements too.

That's the kind of knowledge that lets you stick a 180mm f/4.5 lens on a 4x5, shoot a 3/4 posed tight head shot, head tilted down, and get just the pupils of both eyes and the tip of the chin in sharp focus while shooting at f/4.5, or put 65/5.6 Super Angulon on the camera and get the entire floor of a huge room in sharp focus, perhaps only stopping down to f/8 to compensate for light fall-off in the corners. A DSLR, no matter how many pixels or how much post-processing, can't do either.

Some lenses tend to exhibit more diffraction effects than other lenses at the same f/stop. I suspect that this is caused by the type of bokeh they have; whether the light from a point light source (distant street light) is concentrated in the center of a spot fading into a soft edge or is concentrated in a brighter rim of bright area.

 

[Frank Petronio]

The practical application is that while f/5.6 provides a "nice" comfortable range of depth of field for most 35mm shots with a 40mm lens, it is an extremely shallow depth-of-field aperture to use on a 135mm lens on a 4x5. So to shoot 4x5 at handheld speeds requires more care with your focusing than you might use with 35mm -- and the same scene will look different between formats, everything else being equal.

 

[Augenblick]

DOF large format

DOF large format

Hum...complex. Thank you all for your considered replies.

 

[Al Kaplan]

It's always a peasure to thoroughly confuse a new large format user ;-) but there are books on the subject, and I think that once you digest what's been written here things will fall into place in your head.

 

[Benjamin Marks]

Don't overthink this

Don't overthink this

Hum...complex. Thank you all for your considered replies.

Yeah. I've been involved in more than one thread where things quickly went sideways on this question. There are a lot of variables. Look. Don't overthink it. The whole "apparent depth of field" thing, in my experience, really doesn't affect people's perceptions at the print sizes most people make (e.g. up to 11x14 inches). I haven't run the numbers, I am speaking purely subjectively. In general, a 135mm lens @ f:8 is a 135mm @ f:8 regardless of the "film size". This is, of course, technically incorrect as many folks will no doubt chime in and point out.

But there are real world differences in how you use these lenses/formats. Your 135mm lens in LF terms is on the wide side of normal for the format (normal = 150, 180) . And the 135's DoF is greater than a 210's dof at a given aperture and less than a 90's--and so on. You will probably be using a tripod and your lens is probably only f:5.6 wide open. Your depth of field will not be the same as a 40mm lens in 35mm terms -- only the angle of view will be approximately equivalent. This is one of the reasons that folks often have to stop down lenses quite a bit in LF photography, which leads to slow shutter times, which leads to tripods, sore backs, hernias, and much bad language.

The only answer: feed that lens some film! In a week of experimenting you'll have a better sense of what that lens does than paying attention to my bloviating.

Enjoy! LF is a mind-blowingly different world.

Ben Marks