[Blue]

Kidding about what? The Sigma 12-24mm is a Full-frame lens, and AFAIK the widest rectilinear lens available for full frame.

Or the 20mm? OK, I'm sure it's not "L" quality, but I'm sure someone somewhere has been able to take a decent picture with one. And it's alot cheaper than the 24L (and wider )

Sigma | Super W/A 20mm f/1.8 EX Aspherical DG DF RF AF | 411101

What about the Sigma 10-20mm F4-5.6 EX DC HSM? Based on reviews and discussion, it seams to have a stronger following.

[Gooshin]

What about the Sigma 10-20mm F4-5.6 EX DC HSM? Based on reviews and discussion, it seams to have a stronger following.

Seems Dr.Blue doesnt bother reading some of the more intresting threads on these forums...

3rd post down, remember, we are talking about Full Frame. The sigma 12-24 is a FF lens, providing a full image at 12mm.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/31629-da-lens-...-thread-4.html

[egordon99]

We're talking about Full-frame compatible lenses. The 10-20's image circle will NOT cover a full-frame sensor, so you will see severe vignetting when used on a FF body.

The 12-24mm is FF compatible, that's why I mentioned it in my post on FF and wide angles.

What about the Sigma 10-20mm F4-5.6 EX DC HSM? Based on reviews and discussion, it seams to have a stronger following.

[Blue]

Seems Dr.Blue doesnt bother reading some of the more intresting threads on these forums...

3rd post down, remember, we are talking about Full Frame. The sigma 12-24 is a FF lens, providing a full image at 12mm.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/31629-da-lens-...-thread-4.html

That's because I recalled Lowell Goudge testing it on film with his PZ-1. But as it turns out, it doesn't vignette from ~13mm up. So perhaps you should read up more yourself.

[egordon99]

OK, but 12mm is still wider than 13mm

That's because I recalled Lowell Goudge testing it on film with his PZ-1. But as it turns out, it doesn't vignette from ~13mm up.

[Gooshin]

actualy as my experience (link) showed, its more like 14mm, and pushing it

because at 14mm the vignetting disappears in my viewfinder, but i know the image view in my Super Program is like.. 95%? i think, or 94%, so the final image will be larger and will probably vignette a bit as well, so really, 14.5~15 is playing it safe.

also, thats without a lens hood!

check out the lenshood shot:

[Blue]

OK, but 12mm is still wider than 13mm

It had been a while since I had read the review and forgot about him stating it vignetting below the 13mm. With the APS-H being speculated as a future sensor size for Pentax, this could change this game a little.

---

Last edited by Blue; 11-07-2008 at 11:50 AM.

[Venturi]

Can someone explain to me how DOF changes with the sensor size, please?

I grasp how a 50mm lens designed for APS-C vs one designed for full-frame would exhibit shallower(right?) DOF as the lens optics are different.

But I'm struggling to understand how the DOF would differ mounting an FA50/1.4 on my K20D vs my PZ-1. The same "image" is being projected back to the film/sensor plane, the only difference is the K20D's sensor isn't big enough to catch all of it. So you end up with a "cropped" version of the same optically identical image.

Am I missing something? If so it wouldn't be the first time I deserved the reference "he's a bit thick in the head, ain't he?".

Thanks!

[Gooshin]

Can someone explain to me how DOF changes with the sensor size, please?

I grasp how a 50mm lens designed for APS-C vs one designed for full-frame would exhibit shallower DOF as the lens optics are different.

But I'm struggling to understand how the DOF would differ mounting an FA50/1.4 on my K20D vs my PZ-1. The same "image" is being projected back to the film/sensor plane, the only difference is the K20D's sensor isn't big enough to catch all of it. So you end up with a "cropped" version of the same optically identical image.

Am I missing something? If so it wouldn't be the first time I deserved the reference "he's a bit thick in the head, ain't he?".

Thanks!

it only changes if you decide to look it from a "relative to" perspective

50mm on APS-C has the FIELD OF VIEW of 85mm on film

so lets say you have a 50mm on your APS-C camera and you want to photograph your friend 10 feet away, if this was a FF camera, your friend would fit in the frame and you would take a picture, but your friend doesnt, because you only SEE the equivalent to 85mm

so you have to take a step back and increase the distance between you and your subject in order to produce the same framing with your APSC camera as with a FF camera given your 50mm lens

depth of field is a product of focal length, aperture, and distance to your subject!!

being further away from your object given the same lens, will increase the depth of field for any given aperture.


but again, this is only true if you use the "compared to" logic. Because a 50mm lens at X aperture and Y distance will produce Z depth of field no matter which format its mounted on.

[Gooshin]

I, sincerely, do have to correct you here.

The magnification is a combined function of focal length and view finder magnification. It is by convention only that with a focal length about 50mm, on a 35mm camera, the resulting magnification is about 1. On a camera like the Pentax LX, you can actually change this.

The convention isn't even well founded. It is just a compromise.

The human's field of view approaches 180° and a 12mm focal length would be more appropriate.

On the other hand, the FoV for sharpest vision is a few degrees only and 200mm focal length would be more appropriate.

45° is the compromise adopted by convention, i.e. 50mm for 35mm film. But it is a convention for view finder makers only.

viewfinder magnification does play a role, thats why the general area is 45-55 when talking about "human vision".

and again, there are TWO parts to this statement that people either dont get, or mix up.

that is field of view, and magnification.

your latter words describe the field of view conventions.

what i am talking about is magnifiation-compared-to-human-vision

with 55mm being roughly 1:1

anything less than 55mm will produce an image that would appear SHRUNK compared to your normal human vision.

anything greater than 55mm will start to bring what you see closer to you.


this is why, in theory, it is best to have the largest format available, because that means for any given focal length you can see more!

granted, sometimes you dont want to see more, and 600-1000mm telelenses cost alot of money,

but i think digital sensor development will compensate in the future, and people will be able to crop to their hearts content without much loss in print-quality.

[georgweb]

@Venturi:

Venturi said:
I grasp how a 50mm lens designed for APS-C vs one designed for full-frame would exhibit shallower(right?) DOF as the lens optics are different.

Gooshin answered:
Depth of field is a product of focal length, aperture, and distance to your subject!!

And I might add, of the COC (which has the very appropriate name 'circle of confusion' :-)

Venturi,
lens optics is another fascinating subject, but it does not play a role in here. In fact, if you have a look at DOF charts, these contain always the factors Gooshin has mentioned. The COC (which in this regard equals film or sensor format) is not mentioned in these tables cause it is being regarded as constant. And that's it. Other than that, the DOF is always the same. The only design features from the lens that play a role here is the focal length and the aperture, and these two are independent of sensor format.

There is a huge thread about DOF on this forum also, but maybe a shortcut would be to read Bob Atkins take on 'digital' DOF.

If you scroll to the bottom of his article you will find a photo of a 'full-format' lens scale and the correction factor for APS-C. In this special case (which is counterintuitive at first glance), it comes out just like you thought it would be, the APS-C-scale would have to show a shallower depth of field.

Best, Georg (the other)

[Venturi]

Okay, I think I got it now...

The whole DOF of FF vs APS-C argument has to do with subject difference as it relates to capturing the same identically framed image.

If I were to use my FA50/1.4 on my PZ-1 to take a photo of my cell phone filling 90% of the frame; in order to capture the same exact image on my K20D with the same lens & aperture I would have to move the camera backwards from the subject. And that movement backwards is what changes the DOF of the resultant "print". In essense it is no different from switching between 120 and 220 film on a MF film camera.

Am I on track now? Or did I veer off track into yet another circle of confusion?

[Gooshin]

Okay, I think I got it now...

The whole DOF of FF vs APS-C argument has to do with subject difference as it relates to capturing the same identically framed image.

If I were to use my FA50/1.4 on my PZ-1 to take a photo of my cell phone filling 90% of the frame; in order to capture the same exact image on my K20D with the same lens & aperture I would have to move the camera backwards from the subject. And that movement backwards is what changes the DOF of the resultant "print". In essense it is no different from switching between 120 and 220 film on a MF film camera.

Am I on track now?

you nailed it.

[Blue]

@Venturi:



And I might add, of the COC (which has the very appropriate name 'circle of confusion' :-)

Venturi,
lens optics is another fascinating subject, but it does not play a role in here. In fact, if you have a look at DOF charts, these contain always the factors Gooshin has mentioned. The COC (which in this regard equals film or sensor format) is not mentioned in these tables cause it is being regarded as constant. And that's it. Other than that, the DOF is always the same. The only design features from the lens that play a role here is the focal length and the aperture, and these two are independent of sensor format.

There is a huge thread about DOF on this forum also, but maybe a shortcut would be to read Bob Atkins take on 'digital' DOF.

If you scroll to the bottom of his article you will find a photo of a 'full-format' lens scale and the correction factor for APS-C. In this special case (which is counterintuitive at first glance), it comes out just like you thought it would be, the APS-C-scale would have to show a shallower depth of field.

Best, Georg (the other)

That change is because that the APS-C camera has to be farther back to get an equivalent FOV. If both cameras with the same were on a tripod at the same spot with the same aperture settings, the DOF would be the same. There are some situations when field of view may not matter in a FF and APS-C. For example, when using a 105mm Macro lens to photograph something relatively small the lens will probably positioned the same way regardless of the format.

In a nutshell, DOF is about controlling the known variables. The variable between or among different format is camera position. Gooshin gave a good summary of this in his post.

Edit: I was writing this when Venturi posted above.

[Venturi]

you nailed it.

THANK YOU!
I've been trying to wrap my brain around how somehow the DOF could possibly change just by changing the film/sensor size when everything else remained constant - and my ears were about to start bleeding.
I guess I just missed the entire part of the discussion relating to distance to subject changing to get a "carbon copy" image. *phew*