A question for the Gurus

Conventional wisdom has it that the longer the focal length, the shallower the depth of field: thus on a 35mm body, the DOF for a 28mm lens (at any given aperture) is greater than for a 200mm lens (at the same aperture).

OK, how does this translate to medium format lenses?

That is, for example, does a 200mm focal length (35mm) lens have a shallower DOF than a 200mm (MF 6x7) lens, or do both have the same DOF? :

Also, does the rule of thumb shutter time (for hand-held exposures) should be no less than 1/focal length apply equally to both a, say, 6x7 and 35mm lens, so that for both a 200mm 6x7 format lens and a 35mm format lens, the handheld shutter speed should not exceed 1/200 sec? :

A Google search is quicker than finding a guru.

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This link is maybe of use. Note actual focal length rule.

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Another article.

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Best regards

Thanks .

The link explained it well.

The last link will commit me to fire and brimstone......where is my Pentax FF DSLR and 31mm Ltd?

Best regards

Don't forget that any 80mm lens has the same DOF as any other 80mm lens.

What happens with format is that on 6x6cm you get the field of view of a standard lens with 75/80mm. Thus, your standard lens has less DOF than a standard lens for 35mm film cameras or APS-C DSLRs.

It's all connected and logical once you've clicked with the idea that DOF depends on magnification. For a given magnification, all lenses say at f8 have the same DOF.

To understand that, think of taking a photo of a small ornament. A 50mm macro is used to take the image. Now we decide to use a 100mm macro to take the same image, but 100mm lenses have less DOF? Yes, at the same distance, but to fit our subject in we have to move twice as far away, thus restoring the DOF.

There are lots of rules of thumb that cover these sort of issues that are better suited to the practicalities of taking photographs, but they are not always strictly correct. That bit is the small print...

John,

Interesting that you should use this particular example, as I have noticed (when taking a shot of a particular subject using these very focal length Macro lenses) that, when using the 50mm Macro lens (35mm film body), I get a much deeper DOF (more background in focus) than I do when using a 100mm Macro lens (same body, same subject, both images filling the frame with the same subject).

My off the cuff impresion when using a 200mm 6x7 lens on a 6x7 body (AP6x7) is that it acts more like a 200mm lens than a 100mm lens on a 35mm body (as far as DOF is concerned)

It's all incredibly complicated and related to magnification, but the simple thing is to remember that for most purposes a longer focal length has less DOF.

The main point though is that the perspective is different, becauee of the change of viewpoint to a more distant one.

Now this is a very pertinent insight.

So often one hears the phrase, "Oh, you can 'zoom' with your feet."

Well, of course, you can't really, since the perspective changes as you move relative to the subject.

Also the new position is often inaccessible for one reason or another...
I find standing on a racetrack to get closer to the cars is frowned upon - even if the damn things haven't moved by the time you've walked there. (Walking nearer to the moon has also proved slightly beyond me)

Moderate zooming with the cropping tool is often a reasonable option - even with only 6MP - though I prefer to get it looking right in the viewfinder if possible.

My understanding is that the shutter speed (Hand Held), should be at least equal to the focal length of the lens - well it works for me

This is a moot point, but if we believe the Pentax handbooks it is actually format-related.

So, for 35mm film cameras, 200mm lens = at least 1/200 sec

For APS-C DSLR 200mm = 200 x 1.5 = at least 1/300 sec

Therefore, for 6x7cm. 200mm lens (and I haven't got a table handy)is hand holdable in theory at slightly slower speeds than 1/200 sec

This actually fits well with experience of Rolleiflexes (6x6cm) where the standard 80mm lens can be happily hand held at 1/30sec, the same as was advised for standard lenses on any format for many years.

So, the summary is that the hand holdable speed is dependent on magnification, not focal length, but focal length is the useful rule of thumb that tends to hold together well enough.

Thanks, John: your suggestion sounds good to me.