Medium Format and Depth of Field

[k_michael]

I recently starting using an RZ67 Pro II. For the last 20 years, I've

been using 35mm. My question has to do with depth of field (DOF). Is

DOF a more critical thing to be observant of with medium format

cameras? It apears to me that with my RZ, I do not get the DOF

shooting at f/8 and a 110mm lens as I would with my Nikon F100 and my

50mm lens.

[maury_cohen]

Exactly Kevin.

DOF is relative to the focal length of the lens, not the film format.

In reverse, many digital cameras, with their relatively short focal lengths have inherently great DOF. Not always a blessing!

[david_goldfarb]

That's how it works. Consider selective focus with super-smooth backgrounds at moderate focal lengths as a thing you can do with the RZ that you can't do with 35mm. It's part of the medium-format "look." If you need to stop down more and don't have enough light, the grain penalty from using a 400-speed film is considerably smaller than the advantage of the larger film area.

[terry_dvorak]

In other words, a 110mm lens used on an RZ has the same dof

as a 110mm on your Nikon. The bigger the format, the more this

is a problem (with 8x10 a "normal" lens is ~300mm), which is

why movements are increasingly of value as format size

increases.

[garry edwards]

Well, I sort of agree with the answer above. DOF is affected by a number of factors but if all else (shooting distance, magnification, C.O.F.) are equal then it comes down, not <b>strictly</b> to focal length but to <i>effective</i> aperture (not f stop). <br>As you will know, the f-number is simply the focal length of the lens divided by the effective, or actual aperture - so f11 on a 50mm lens has an aperture of 4.5mm and f11 on your standard 110mm lens has an aperture of 10mm.<br>So, to get the same DOF with your 110mm lens you need to have the same aperture as on your 50mm lens, which means that f11 (4.5mm)on a 50mm lens = f24.44 on your 110mm lens.<br>I have a very good DOF chart in Excel, let me know if you'd like it and I'll email it to you.<p>Hope this helps.

[andre_oldani]

To see the different DOF results of the various formats you can try the DOF calculator I found on the web (by Michael Gillett). Go to http://www.alpavision.ch and then to "alpaca" and the DOF calculator link. The site is about the new ALPA cameras which are plain wide angle finder cameras with distance estimation and DOF is important.

 

Fill out the form and change then the format type or other variables and watch the results. :-)

 

Best,

André

[pete_andrews]

Depth-of-field is affected by both the focal length of the lens AND the format size, for any given aperture and subject distance.<br>That's why consumer digital cameras, with their tiny 6mm x 8mm sensors, have an incredibly large DoF. It's so large, in fact, that one of the major criticisms aimed at them, is that you can't easily control DoF to isolate the subject.<p>DoF decreases as format size increases, and for a given angle of view and subject distance, it's close enough to say that if you double the format dimensions, you also need to double the numerical lens aperture to get the same depth-of-field.<p>A 110mm lens on 6x7 is roughly the same as a 50mm lens on 35mm, if you crop the 35mm frame to the same ratio as 6x7, but the 6x7 frame is 2.3333 times bigger.<br>So if you get a certain amount of DoF at f/8 on 35mm, then you'd need to stop the lens on the 6x7 down to (8 x 2.333) nearly f/19, to get the same effect.<p>The comparison with a consumer digicam is quite frightening. The digital camera gives the same DoF at f/2 as our 6x7 and 35mm examples above give at f/19 and f/8 respectively.<br> Yikes!

[gene crumpler]

Best rule of thumb for MF. Set up the focus range with the DOF indicators, then stop down at least two stops. Three stops if you are trying for a 16x20 print.

[albertini_f]

Hi,

I assume that two lenses of different focal length at the same aperture will give a different out of focus �look�. So assuming that I shoot with a 100mm MF lens and a 100mm LF lens, using the same aperture for both, I�ll have the same �look� effect of the image out of the plane of focus; Am I wrong here?

I�m not talking about the DoF itself, but just the �look� the softness and smoothness of the out of plane of focus images, like the look that makes the LF images so recognizable.

Do also brand-makers or lens-camera construction-size affect that Out of Focus �Look�?

Thank you

[garry edwards]

<b>Reply to Albertini</b><p>In theory, a lens is a lens is a lens, and there should be no difference between a lens intended for MF and a lens of the same focal length intended for LF, but the theory falls down even when making direct comparisons, for example when comparing Hasselblad and other MF lenses, and Leica/Nikon with other 35mm lenses.<br>There is actually quite a big difference between MF and LF lenses - most MF lenses (but not those intended for tilt/shift) only produce an image circle just large enough to cover the required format when set at infinity, LF lenses have to have a much larger image circle because they need to cover the film area even when a large amount of shift is being used, so obviously their design is different.<br>I think it's probably true to say that much of the 'special look' of LF is due partly to the amount of fine detail contained in the larger images and partly to the effect of the camera movements used.

[mel_brown1]

"Depth-of-field is affected by both the focal length of the lens AND the format size, for any given aperture and subject distance."

 

Pete, that is not an entirely accurate statement because it does not include "for a given print size viewed from a given distance." Note that the DOF formulae do not consider film format. They do, however, consider the CoC (Circle of Confusion), a somewhat arbitrary dimension that we conveniently alter for different film formats. We do that on the assumption that we don't enlarge a larger negative as much as a smaller one to achieve a given print size. Also, as I recently noted in another thread, even if we enlarge the larger negative to the same degree (producing a larger print), we tend to view that larger print from a greater distance, normalizing any perception of fuzziness.

 

Changing the CoC for each film format is a reasonable action, so long as the assumptions above are correct. However, it is good to keep in mind that the reduced DOF apparent in prints made from larger format cameras is due entirely to the longer lenses typically used with those cameras, and has nothing to do with film format itself.

 

To say it another way, consider using a 4x5 camera to shoot first one image on 4x5 film, then another on 120 film, using a rollfilm adapter. There is obviously no way for the DOF to change merely by changing film size.

 

It is only when we magnify images to different degrees (in order to produce a common print size) or when we view different print sizes from the same distance that we would perceive a different DOF when comparing the two prints.

 

With the f-number and focused distance held constant, longer lenses produce ON FILM less DOF than do shorter lenses, regardless of film size.

 

Kevin, your observation is spot-on. The longer lens on your RZ reduces the DOF by a bit over 2 aperture steps. If you were to stop it down to somewhere between f/16 and f/22, it would produce the same DOF as your Nikon with its 50mm set to f/8. That is the price we have to pay in exchange for a larger negative.

[albertini_f]

Dear Garry Edwards,

Thank you for your fulfilling post.

Always I have been puzzled by that phenomenon.

I simply visually compared prints made my 200mm Nikon, 200mm P67 and 210mm Schneider LF; and the out of focus �look� is very different.

If true that this effect differs even between brand-makers of same camera size negative,

this phenomenon could also affect the choice of one brand lens over another.

Regards,

[pete_andrews]

OK Mel. I omitted to say "for a given angle-of-view, aperture and subject distance, <i>and for a given final print size and viewing distance.</i>", but I had to leave something out for other people to pick me up on, didn't I?<br>And didn't we come to the same conclusion, that f/19 on a 6x7 camera with a 110 mm lens gave the same DoF as a 50mm lens @ f/8 on 35mm?

[david_senesac]

I made the switch 4 years ago from many years of 35mm to a Pentax 6x7. A friend of mine doing likewise chose the LF route despite the associated difficulties in order to reduce DOF issues with movements. Even though I was aware of the DOF limitations, initially I sometimes was too close to landscape foregrounds such that the resulting near focus gave noticeably unacceptable results. With 35mm I was used to using the infinity stop on lenses for most situations while with the 6x7 I have learned to move the focus back from infinity judged upon the frame contents and thus improve the ability to get closer. For instance often in landscapes at lower elevations with distant hills the crispness of those subjects of a frame are not too sharp because the distance of the air itself reduces such. And of course what is the distance of the important subjects of an image? By shooting more intelligently there is much to be gained. In any case I can't shoot as close as with 35mm for the same angle of view and would quickly agree that such a composure often produces the finest frame. Hence a definet edge to LF users. But I've have instead still found ways to provide fine enough foregrounds by being further back as these last few years of work might arguably be my best. -dave

[mel_brown1]

Pete, you not only omitted important information, but you also wrote, "Depth-of-field is affected by ... format size," and "DoF decreases as format size increases,..." both of which are patently untrue statements.

 

And no, we did not come to the same conclusion. I stated to Kevin, "The longer lens on your RZ reduces the DOF by a bit over 2 aperture steps." It's the longer lens that does it, not the format. You continue to cloud the issue by injecting film format into the equation.

 

You seem annoyed with me for pointing out your error. It was not my intention to nitpick you, but many people come to this forum to learn how things work. It's not fair that they be misled by classic half-truths. You are not alone; many before you have perpetuated the same misinformation.

 

I make mistakes too, and if you catch me in one, please feel free to point it out to me. I hope to be big enough simply to admit my error and get on with my life.

[photostudent19beth_davies]

this is really another question...at least one person that has already responded might be able to answer it.

 

will the depth of field in a 10x12 print and viewed from the same distance appear to be more when printed from a medium format negative than a 35mm, as the medium format neg requires less magnification and so will have smaller circles of confusion.

??????

 

For example, a scene shot with a mf camera and the same scene shot with a 35mm, given the same aperture, and angle of view.

The distance of focus would be different, and the length of the lens would be different for each format. The absolute image size on the negative would be bigger in the mf. But the relative image

size/format would be the same...and so the angle of view would be the same.

effectively what i have described is shot in a 35mm camera with a 50mm lens at a distance of x and the same shot with a mf camera at a distance of (x - a bit) to compensate for the larger format.

 

basically the standard lenses on each camera framing the same shot at the same aperture.

 

Of course the perspective would be different in each shot. But i know that this is a function of the distance rather than focal length, and besides, background blur as a result of perspective is simply magnification and so is irrelevant in determining depth of field.