[falconeye]

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In other words, do we really need a 55mm f/0.92 to simulate the appearance of an 85/1.4?

Yes. As I tried to explain in too many threads now.
DoF only depends on FoV and physical aperture in mm. Whatever be the sensor size or focal length (of course, both would have to match the FoV).

(It is a different question if you use both lenses on the same body and move closer with the 55mm to get the same magnification.)

[lbam]

Unscientific sounds good, I will try it.

I just received a mail from my dealer. The DA*55 is in stock. I pic it up on tuesday morning and will report on tuesday evening.

Rainer

Get it yet? lol

[TKH]

Get it yet? lol

: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/51392-da-55mm-vs-fa43mm.html

Rainer

[nixcamic]

Yes. As I tried to explain in too many threads now.
DoF only depends on FoV and physical aperture in mm. Whatever be the sensor size or focal length (of course, both would have to match the FoV).

(It is a different question if you use both lenses on the same body and move closer with the 55mm to get the same magnification.)

Don't you mean DoF only depends on distance to subject and physical aperture in mm?

[SteveM]

For those of us in Canada....it's now on ebay for $749.99 US...
Brand New smc PENTAX-DA* 55mm F1.4 SDM Lens In Stock! - (eBay.ca item 120380209358 end time 19-Mar-09 19:25:18 EDT)

[Mister Guy]

Don't you mean DoF only depends on distance to subject and physical aperture in mm?

Well he should mean distance, focal length, and aperture, assuming sensor size is constant.

[nixcamic]

Well he should mean distance, focal length, and aperture, assuming sensor size is constant.

Relative aperture, which is given in f stops, is affected by focal length, but only because it's the ratio of aperture size to focal length. It's this way so you dont have to do logrithms when changing lenses to keep a constant exposure.

Absolute aperture, which it would be in mm, is the actual measurement of the aperture opening, in some normal constant measurment system, like mm. Any lens at any given absolute aperture will have the same DoF as any other lens at that absolute aperture focused at the same distance, regardless of focal length or sensor size.

[falconeye]

Relative aperture, which is given in f stops, is affected by focal length, but only because it's the ratio of aperture size to focal length. It's this way so you dont have to do logrithms when changing lenses to keep a constant exposure.

Absolute aperture, which it would be in mm, is the actual measurement of the aperture opening, in some normal constant measurment system, like mm. Any lens at any given absolute aperture will have the same DoF as any other lens at that absolute aperture focused at the same distance, regardless of focal length or sensor size.

nixcamic, what you said is correct, almost. As those DoF discussions tend to never stop, let me make this precision:
Your argument holds true for a given field of view (FoV) only. What I said and you forgot to say. E.g., if you crop into an image, the DoF changes (because the permissable circle of confusion (CoC) does -- please, all refrain from answering on this remark -- al least, I'll do... ).

[leeak]

Yes. As I tried to explain in too many threads now.
DoF only depends on FoV and physical aperture in mm. Whatever be the sensor size or focal length (of course, both would have to match the FoV).

(It is a different question if you use both lenses on the same body and move closer with the 55mm to get the same magnification.)

So would it be correct to say a the DOF is the same for DA*55 at 1.4 on a APS-C sensor and FA*85 at 1.4 on a FF sensor?

[falconeye]

So would it be correct to say a the DOF is the same for DA*55 at 1.4 on a APS-C sensor and FA*85 at 1.4 on a FF sensor?

No.
Except if 39 mm = 61 mm ...
Please, go back a couple of posts and read, why:
https://www.pentaxforums.com/forums/493400-post59.html
https://www.pentaxforums.com/forums/493637-post61.html
https://www.pentaxforums.com/forums/494603-post67.html

[Zebooka]

So would it be correct to say a the DOF is the same for DA*55 at 1.4 on a APS-C sensor and FA*85 at 1.4 on a FF sensor?

DoF ( DA* 55 at 1.4 on APS-C ) = DoF ( FA* 85 at 2.1 on FF)

[nixcamic]

I was going to forget about this all, but since everyone else keeps talking about it I figure I might as well get into it as well. Your post raises a good point, I forgot completely about circles of confusion.

An APS-C sensor with the same amount of megapixels as a FF sensor (ie a higher pixel density) has significantly smaller circles of confusion and significantly less DoF.

I did some calculations, and a 35mm f/1.4 lens on a 12mp APS-C sensor does have very close DoF to a 50mm f/1.4 lens on a 12mp FF sensor, but it still is different. within 3', the difference between the two is never more than a fraction of an inch, up to around f/5.6 and by then, you can just open the lens up a stop more to make up for it (assuming your lens goes faster than f/5.6) at distances above 4', the difference in DoF does start to climb quite quickly, getting up to 102 feet difference at f/1.4 in DoF by the time you're focused out at about 100'. I realize this is a drastic and horrible difference for some people, and does indeed matter to them, but most of us need to ask ourselves, how much better would my portraits be with 1/2" less DoF? Can I even see the extra inch of DoF on that full body shot? There will of course, be the occasional instance where it would be an improvement, but if its not $3000 better, what's the point?

As the megapixel war rages on, DoF keeps getting shallower and shallower, at 10MP APS-C we already have the same DoF as most 35mm films at equivalent (1.5x) focal lengths.

Hopefully this makes sense, I wrote it far to late at night, I probably shouldn't have written it all, this isn't really the appropriate thread.

[falconeye]

Disclaimer: I don't want to bring this thread OT. So, this will be my last post about DoF in this thread.

As the megapixel war rages on, DoF keeps getting shallower and shallower [...] I probably shouldn't have written it

Didn't I write "please, all refrain from answering on this remark"?

nixcamic, as you write, this isn't the thread. So, to just mimimize confusion and because you spent so much time on it...

For DoF calculations, the circle of confusion (CoC) is traditionally defined to be something like image-diagonal/1730 (*). It reflects the resolution limit of the human eye when looking at the entire image. Where walking closer is nothing but cropping (remember my cropping remark that cropping changes DoF? )

The CoC is much larger than the pixel pitch (like 5x for a K20D). You can ask at the subject distances which still render sharp pixels. But this is not called "depth of field" DoF. You must give it another name. Because DoF is an established term defined independently from digital technology.

Therefore, DoF is independent from megapixel. By definition.

--
(*) For 1730, cf. Zeiss formula - Wikipedia, the free encyclopedia and it doesn't matter if this or any other constant is used. Pentax seems to use 1500 on their lens scales. I recommend 2500 in the digital age.

[Zebooka]

An APS-C sensor with the same amount of megapixels as a FF sensor (ie a higher pixel density) has significantly smaller circles of confusion and significantly less DoF.

I thought this circle is measured in % of frame diagonal. And most of people think so too

[jeffkrol]

Understanding Depth of Field in Photography