A question for the Gurus


Banjo

Link Posted 22/11/2010 - 04:35
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?
Last Edited by Banjo on 22/11/2010 - 04:41

K10D

Link Posted 22/11/2010 - 05:42
A Google search is quicker than finding a guru.

link


link

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

link

Another article.

link

Best regards
Last Edited by K10D on 22/11/2010 - 06:05

Banjo

Link Posted 22/11/2010 - 06:06
K10D wrote:
A Google search is quicker than finding a guru.
link
Best regards

Thanks .

The link explained it well.

K10D

Link Posted 22/11/2010 - 06:15
Banjo wrote:
K10D wrote:
A Google search is quicker than finding a guru.
link
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

johnriley

Link Posted 22/11/2010 - 08:25
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...
Best regards, John

Banjo

Link Posted 25/11/2010 - 00:44
johnriley wrote:
... 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...

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)

johnriley

Link Posted 25/11/2010 - 08:45
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.
Best regards, John

Banjo

Link Posted 26/11/2010 - 00:18
johnriley wrote:
...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.

petrochemist

Link Posted 26/11/2010 - 00:48
Banjo wrote:

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.
Mike
.
Pentax:K5ii, K7, K100D, DA18-55, DA10-17, DA55-300, DA50-200, F100-300, F50, DA35 AL, 4* M50, 2* M135, Helicoid extension, Tak 300 f4 (& 6 film bodies)
3rd Party: Bigmos (Sigma 150-500mm OS HSM),2* 28mm, 100mm macro, 28-200 zoom, 35-80 zoom, 80-200 zoom, 80-210 zoom, 300mm M42, 600 mirror, 1000-4000 scope, 50mm M42, enlarger lenses, Sony & micro 4/3 cameras with various PK mounts, Zenit E...
Far to many tele-converters, adapters, project parts & extension tubes etc.

.[size=11:].Flickr WPF Panoramio
Last Edited by petrochemist on 26/11/2010 - 00:51

mayday

Link Posted 26/11/2010 - 08:50
Banjo wrote:

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?

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
Regards
David

Retired at last - now all that time for photography - you would think: wink:

johnriley

Link Posted 26/11/2010 - 08:56
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.
Best regards, John

Banjo

Link Posted 28/11/2010 - 23:15
Thanks, John: your suggestion sounds good to me.
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