RD-1 Depth of Field

[ducttape]

Forgive this ignorance, as I assume I could probably figure this out given enough time and coffee...

Is the depth of field of any lens on the RD-1 as marked on that lens's barrel, or that of a lens the size of the camera's 35mm equivalent field of view?

In other words: My 50mm lens is marked with a depth of field scale. Is that the correct scale for this lens/camera combo, or do I get the depth as if I used the 75mm lens, which is the field of view offered by the 50mm lens on the RD-1?

(Is that clear?)

Thanks!

 

[hoot]

AFAIK, the depth of field is always in inverse proportion to the size of the medium. I.e., at a given aperture (f-stop), a 6x6 negative will *always* have less DoF than a 35mm negative, and the RD-1 digital sensor, being smaller than a 35mm negative, will *always* have more DoF at that same aperture. Unless the M-mount lenses you are using are specifically made for the RD-1, the DoF scale will thus be wrong, as you will always be getting sharper images than it indicates. However, even with film, I find that these indications are merely guidelines, and that your actual subjective sharpness depends on the size of the enlargement, and often the choice of subject as well.

The good news is that the high DoF enables working at a lower ISO setting, because shooting wide open will be a lot sharper.

 

[ipuustin]

The image projected by a given lens is exactly the same on any camera body, be it a digital body with small sensor or a film body. Thus, RD-1 just crops the image smaller, but the other factors (bokeh, DOF) will be similar to any film body. Remember: the lens doesn't know which body it is attached to! This means that the DOF scale printed on the lens still applies.

I believe that the "crop factor" confuses a lot of people -- it only means that the image is cropped. The focal length and aperture (which contribute to the DOF effect) stay the same. The confusion arises from the fact that people use 30mm lenses as 50mm lenses on a digital body, but the 30mm DOF is wider than the DOF of a real 50mm lens.

Hope this helps!

 

[hamsong]

Hi David-

Actually, the DOF with the R-D1 is the same as is marked on your lens. The concept of DOF actually can be hard to get one's mind around, but it was a revelation when I figured out that the DOF of all lenses of any given focal length are, in fact, the SAME, regardless of format.

It is thought that DOF is shorter on medium format because a "normal" lens is a 80 in MF and a 50 in 35mm. The reason for this is that the DOF of an 80mm is still the same in both formats, and thus, in order to achive the FOV of a 50mm lens, a longer lens is used.

The thing that is missing from a lot of discussion about crop factors like 1.53x crop of the R-D1, etc, is that cropping a 50mm to be a 75mm FOV doesn't really make it a 75 in that you still have the magnification and DOF of a 50mm. Because one of the factors of DOF is distance from subject (in addition to selected aperture), you actually alter it by being further away from you subject with a 50mm showing 75mm FOV.

A good example of this is how one assumes that f16 will give nearly universal DOF. When using lenses like the 120 Zeiss Macro at close range, even at f16, DOF can be *extremely* shallow.

 

[hoot]

Oops, looks like I got my head up my posterior for a moment there. Sorry. 🙂

Cute avatar, Brad. Haven't seen you around these parts before.

 

[LCT]

the DOF with the R-D1 is the same as is marked on your lens

I beg to differ WADR.
Due to the different circles of confusion, you'll get a good approximation if you use the DoF markings of the nearest faster f stop of your lens i.e. f/5.6 when you choose f/8 for metering for instance.
See http://www.rangefinderforum.com/forums/showthread.php?p=128694
FWIW
Best,
LCT

 

[zetablues]

Yes, the DOF is a function of a lens (depends on focal length and f-stop) and object distance, has nothing to do with the digital crop factor (FOV).
Regards

 

[davidcreede]

If you were usuing a film camera I could understand your dilemma...

If you were usuing a film camera I could understand your dilemma...

But you are using a digital camera which has free film and no processing delays and thus you can instantly test it for free. So enough with the back and forth about who is right, (I happen to be of one camp, but which one is irrelavant) go get the tape measure out of your dad's sewing kit, or your mom's toolbox and go test the depth of field. If you are feeling magnanimous you could even get back to us with the pictures, specs, and results.

 

[Hektor]

What you are calling the "depth of field" when applied to an image, is actually the 'depth of focus' and is indeed the same for any lens of a given focal length and aperture, regardless of the camera it's fitted to, - and so the dof lens scale does apply.

However the recorded image is usually viewed as an enlarged print (or similar reproduction) and therefore the degree of enlargement will affect the apparent sharpness.

The dof lens scale is calculated witha constant circle of confusion regardless of intended enlargement, - and is often thought suitable for about 5-10 diameters of enlargement.

A competent careful practioner with excellent equipment can achieve satisfactory images from 35mm with enlargements of 20 to 40 diameters. These will definately show up the inadequate mathematical assumption in the chosen figure for the "circle of confusion". Using the next lower aperture marks on the dof scale, when intending big enlargement, alleviates this phenonemon.

Basically, with a small sensor/image area, it is safer to assume a smaller figure for CoF when intending to employ a greater enlargement, so as to maintain a satisfactory enlargement for final viewing.

Finally, it's generally better to use the shortest suitable focal length taking lens because it has greater depth of field than a longer one.

 

[richard_l]

Forgive this ignorance, as I assume I could probably figure this out given enough time and coffee...

Is the depth of field of any lens on the RD-1 as marked on that lens's barrel, or that of a lens the size of the camera's 35mm equivalent field of view?

In other words: My 50mm lens is marked with a depth of field scale. Is that the correct scale for this lens/camera combo, or do I get the depth as if I used the 75mm lens, which is the field of view offered by the 50mm lens on the RD-1?

(Is that clear?)

Thanks!

The region of acceptable focus for a 35mm image is traditionally determined by what appears to be in acceptable focus in an 8x10" enlargement at an "normal" viewing distance. If the 8x10" criterion is used for an R-D1 image, then, say, a 50mm lens on the R-D1, will have about the same DOF as that of a 60mm lens on a 35mm camera. The way to compute this is to multiply the nominal focal length of the lens by the square root of the crop factor. For a 1.5 crop factor, this will be about 1.2. I proved this mathematically in another thread a few weeks ago and don't want to repeat it here.

Richard

 

[richard_l]

For a given lens on the R-D1, the DOF for a given f-stop F will be the same as the DOF for the f-stop F/1.5 (F divided by the crop factor) for the same lens on a 35mm camera. This is approximately one stop larger aperture. Therefore, as others have suggested, just use the DOF scale for one stop larger aperture in order to get a rough idea of the DOF on the R-D1.

Richard

 

[richard_l]

One more comment. The focal length of a lens does not change when it is mounted on the R-D1. The focal length of a 50mm lens is always 50mm whether it is on a 35mm camera or the R-D1 or just sitting by itself in a shelf somewhere. The same goes for a lens of any other focal length. The image on the R-D1 sensor would (theoretically) be exactly the same as if 35mm film were used (with the same lens and f-stop) and the negative cropped to be the same size as the R-D1 sensor. It's all really very simple if you just think of it that way.

Richard