Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
12-24-2014, 03:40 PM   #1
Cary Berman
Guest




Crop Factor from a 645 to a K3 APS-C

Please help me figure out the Crop factor between Pentax 645 lens and K3 APS-C. Also do you know if any adaptor is required to mount the lens? Last question what features work in this case such as auto focus?

12-24-2014, 03:48 PM   #2
Pentaxian




Join Date: Sep 2013
Photos: Albums
Posts: 817
You'll need a 645 to k adapter. The crop factor is mostly meaningless. 75mm on the 645, is approximately equivalent to 35mm on apsc. When mounted on apsc, it will be a telephoto lens (similar to the FOV of the 77 fa limited). You'll be limited to manual focus, and no info aside from focal length will be reported in the exif (but only if shake reduction is enabled)
12-24-2014, 03:53 PM - 2 Likes   #3
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 40,992
QuoteOriginally posted by Cary Berman Quote
Please help me figure out the Crop factor between Pentax 645 lens and K3 APS-C. Also do you know if any adaptor is required to mount the lens? Last question what features work in this case such as auto focus?
So you want to use a 645 lens on your K-3? You can use this adapter, but you wouldn't get AF and you'd have to stop-down meter as per these instructions.

The APS-C vs 645 film crop factor is roughly 2.4x . However, focal length is an optical property unaffected by sensor size. Thus, if you mounted something like a FA-645 55mm on a K-3, you'd get the same exact field of view as from any other K-mount 55mm lens. It would act as a short tele on the K-3 rather than a wide-angle on a 645 body.

Thus, the only practical scenario in which adapting a huge medium format lens would make sense is super tele photography. If only own a long 645 lens like a 400mm or 600mm, rather than buying one with a K mount, you could simply adapt it.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

12-24-2014, 03:54 PM   #4
owl
Forum Member




Join Date: Jun 2008
Location: Upstate New York, USA
Posts: 92
FLx1.5, simple as that.

No auto focus, but focus confirm will still work with manual focus.

12-24-2014, 04:21 PM - 1 Like   #5
Pentaxian
maxfield_photo's Avatar

Join Date: Mar 2011
Location: Cincinnati, OH
Posts: 1,216
QuoteOriginally posted by Adam Quote
Thus, the only practical scenario in which adapting a huge medium format lens would make sense is super tele photography. If only own a long 645 lens like a 400mm or 600mm, rather than buying one with a K mount, you could simply adapt it.
I can think of another. I've often thought about mounting a 645 or 67 lens on a tilt-shift adapter so that I could take advantage of the larger image circle. I've yet to try it though, so there may be some factors that I'm not taking into consideration.
01-12-2015, 02:47 PM   #6
Dick Smith
Guest




Would be photographer

I have just adapted a 645 600mm lens to my K5lls and it works like a 900mm. It does take an adapter (645 to K). This outfit is manual focus and I use a Pentax Spot V one degree spotmeter to adjust the exposure. WORKS GREAT!!!
01-12-2015, 07:08 PM   #7
mee
Site Supporter




Join Date: May 2010
Posts: 5,258
QuoteOriginally posted by Adam Quote
The APS-C vs 645 film crop factor is roughly 2.4x . However, focal length is an optical property unaffected by sensor size. Thus, if you mounted something like a FA-645 55mm on a K-3, you'd get the same exact field of view as from any other K-mount 55mm lens. It would act as a short tele on the K-3 rather than a wide-angle on a 645 body.
This is OT but that was thoroughly confusing to me.

Just by using the term 'crop factor' it reads to me as if you are saying the field of view isn't going to be the same - a portion of the viewable image (with FF or larger sensor) is going to be cropped away from what one would see with an APS-C sensor... no? That is, the framing of the actual image is going to be different between these sensors with the same lens. If true, what occurs with this supposed 'zoom' amount? Is there really a zoom amount optically or is this just an illusion based on the sides, top, and bottom being cropped from view?

I've read so many articles and some agree with one method of describing crop factor with respect to field of view and others say something different.
01-12-2015, 08:29 PM   #8
Pentaxian
maxfield_photo's Avatar

Join Date: Mar 2011
Location: Cincinnati, OH
Posts: 1,216
The problem with crop factors and different formats is that there are different ways of understanding the same phenomenon, and one will generally make sense to a given individual, while the other ways of thinking of it seem "backwards". None the less, they are all true.

What Adam said is correct, but "crop factor" also works in reverse. If you took a 55mm 645 lens, mounted it on a 645 film body, it would be a wide angle lens, mount the same lens on the 645D or 645Z (which have a crop factor compared to medium format film) and the lens is a true "normal". Mount the same lens on a 35mm film body, and it becomes a "long normal", and mount it on an APS-C body like the K-3 and it becomes a portrait length lens. The lens doesn't know or care to which body it is mounted. What you see in the viewfinder will change from body to body because you are taking ever smaller pieces from the center of the frame. The focal length remains 55mm on every body, but the lens' role in your camera bag changes.

Now if you could somehow mount a K-mount DA* 55mm on a 645 film body let's say (ignoring for a moment that it would need to mount inside the mirror box to maintain infinity focus), you would still see what to can see if that lens were mounted on a K-3, but the corners of the frame would be completely black, because that lens was not designed to fill the images circle of a medium format film camera.

If you find the concept of "crop factor" confusing, try to find an article that explains it in a way that makes sense to you, and hold on to that knowledge, but at the same time understand that there are different ways of explaining it. Once you have a good strong grasp of the concept, begin to try and understand it from one of the other perspectives.

01-12-2015, 09:32 PM   #9
mee
Site Supporter




Join Date: May 2010
Posts: 5,258
QuoteOriginally posted by maxfield_photo Quote
If you find the concept of "crop factor" confusing, try to find an article that explains it in a way that makes sense to you, and hold on to that knowledge, but at the same time understand that there are different ways of explaining it. Once you have a good strong grasp of the concept, begin to try and understand it from one of the other perspectives.
If it was that simple, I wouldn't have spent part of my evening posting the message above. I thought I understood but I've heard many different descriptions and some of them seem to work opposite of what other people are saying.

I appreciate the response though.. The difference seems to be the field of view DOES change with respect to sensor size -- you just stated that 55mm lens went from wide angle to normal to portrait 'length' simply by the size of the sensor decreasing. The focal length stayed 55mm but the field of view changes. So how can someone say the field of view (that is, the framing) doesn't change -- it does change! Look below:




I guess the issue is the term 'field of view' -- what does that mean here? When I read that I think the amount of 'world' I can get in view. You can get a ton of world in the view with that 55mm 645 lens on a 645 body but not nearly as much on the APS-C body. Or FF to APS-C, As shown in the image above.

Maybe everyone is saying the same thing differently.. but some people seem to be saying things that run completely opposite of others. ah such a strange concept. maybe I don't need to concern myself with it..
01-12-2015, 10:26 PM   #10
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 40,992
QuoteOriginally posted by mee Quote
This is OT but that was thoroughly confusing to me.

Just by using the term 'crop factor' it reads to me as if you are saying the field of view isn't going to be the same - a portion of the viewable image (with FF or larger sensor) is going to be cropped away from what one would see with an APS-C sensor... no? That is, the framing of the actual image is going to be different between these sensors with the same lens. If true, what occurs with this supposed 'zoom' amount? Is there really a zoom amount optically or is this just an illusion based on the sides, top, and bottom being cropped from view?

I've read so many articles and some agree with one method of describing crop factor with respect to field of view and others say something different.
Two lenses with the same focal length used on any given camera will produce the same field of view. The crop factor just refers to the difference between the field of view of say a 55mm lens on 645 film vs its field of view on APS-C.

This article might help shed some light on the whole thing, which I'll admit is one of the most confusing concepts of the digital photography era.

The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

01-12-2015, 10:26 PM   #11
Veteran Member
sterretje's Avatar

Join Date: Dec 2008
Location: Roodepoort, South Africa
Photos: Gallery | Albums
Posts: 3,534
Let me add to the confusion (or take it away)

In the below drawing, the solid vertical at the left of the lens represents the film (or full frame sensor, red; further referred to as film) or APSc sensor (blue; further referred to as sensor). At the right of the lens is our subject (tree) surrounded by some sky and a foreground. For now, forget the red dashed vertical at the left. Note that the focal length is defined as the distance between the focal plane and the lens.

If we take a 50mm lens and place it on a film camera, we draw diagonal lines from the outer of the film through the center of the lens till they 'hit' the subject; those are the red diagonal lines. You can see that the film captures foreground, tree and sky.
If we keep the same position and place the 50mm lens on an APSc camera, we can do the same; drawing the blue diagonal lines from the outer of the sensor trough the center of the lens till we hit the subject. You can see that the sensor only captures the tree.

This matches the picture posted by mee above.



To get the same framing on film as on APSc with 50mm lens, we extend the blue diagonal lines to the left and move the film to the left; we stop when the outer of the film fits exactly in the extended blue diagonal lines. Some mathematics can tell you that you have to move the film 25mm to the left so the distance of the film to the lens is now 75mm. This means that a 75mm lens on film will give you the same field of view as a 50mm on APSc.

Note: The crop factor is totally irrelevant if you stay within one 'system'; focal length is focal length and a physical attribute of a lens.
01-13-2015, 12:10 AM   #12
mee
Site Supporter




Join Date: May 2010
Posts: 5,258
QuoteOriginally posted by Adam Quote
Two lenses with the same focal length used on any given camera will produce the same field of view. The crop factor just refers to the difference between the field of view of say a 55mm lens on 645 film vs its field of view on APS-C.
Thank you for that link but I seem to be missing some key element in this.. The first line you state there seems to conflict with the second.

I mean in the article it states "Another way of looking at it is that the field of view is narrower on APS-C compared to full frame given the same lens"

So the field of view does change? Perhaps, if it does not change, you mean the sensor 'sees' the same FoV with that 645 lens regardless of camera body to which it is affixed, the difference is that the sensor (for instance a K3 uses) is smaller on APS-C and thus that same field of view is contained in a smaller view? Or just that part of that FoV is clipped due to the smaller sensor size?

ahhhhhhhhhhhhh.

I guess I'll read that article over and over again..
01-13-2015, 05:53 AM   #13
Pentaxian
zantaphia's Avatar

Join Date: Mar 2011
Location: Skövde, Skaraborg
Photos: Gallery | Albums
Posts: 486
Hi!

While shooting 3 different formats (APS, FF and 645 digital), I use those simple formulas for calculating approx. FoV between the different formats:


FL 645 digital / 2 ~ FL APS
FL APS * 2 ~ FL 645 digital

FL APS * 1.5 = FL FF
FL FF / 1.5 = FL APS , alternative formula: (FL FF / 3) * 2 = FL APS

FL 645 digital * 0.75 ~ FL FF
FL FF / 0.75 ~ FL 645 digital , alternative formula: (FL FF / 3) *4 ~ FL 645 digital

Example: 100 mm FL on 645 digital is approx. the same FoV as 50 mm FL on APS, and 75 mm FL on FF.
01-13-2015, 07:47 AM   #14
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 40,992
QuoteOriginally posted by mee Quote
Thank you for that link but I seem to be missing some key element in this.. The first line you state there seems to conflict with the second.

I mean in the article it states "Another way of looking at it is that the field of view is narrower on APS-C compared to full frame given the same lens"

So the field of view does change? Perhaps, if it does not change, you mean the sensor 'sees' the same FoV with that 645 lens regardless of camera body to which it is affixed, the difference is that the sensor (for instance a K3 uses) is smaller on APS-C and thus that same field of view is contained in a smaller view? Or just that part of that FoV is clipped due to the smaller sensor size?

ahhhhhhhhhhhhh.

I guess I'll read that article over and over again..
The effective field of view by all means changes; the focal length does not.

Basically, since the K-3's sensor is smaller, it only sees a fraction of what the lens actually covers. With a bigger sensor (or piece of film) you'd be able to see more of the image circle.

Make sure you understand the above before reading on.

-----------

Now, here's what a lot of people get wrong about this whole crop factor thing. Many believe that a shorter lens designed for a larger sensor format, say a full-frame FA* 300mm, would be "equivalent" to a 450mm when used on APS-C, a smaller format.

This is only correct if you compare the field of view of a hypothetical 450mm lens on full-frame to the field of view of a 300mm on APS-C.

People think you automatically get "more zoom" on APS-C, but the reality is that focal length is a fixed optical property of the lens. So, you gain absolutely nothing (in terms of reach) by mounting a full-frame 300mm on an APS-C camera compared to mounting an APS-C only (i.e. DA) 300mm on an APS-C camera. What you do get is a tighter field of view than you would with the same 300mm lens on full-frame, and thus smaller formats are better for telephoto shooting since you don't need as long of a lens to isolate a distant subject in the frame. Conversely, larger formats are better for wide-angle photography since you don't need as short of a focal length to get an ultra-wide field of view.

That's all there's to it- hope I didn't lose you!

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

01-13-2015, 08:02 AM   #15
Pentaxian




Join Date: Dec 2010
Location: Ontario
Photos: Gallery
Posts: 2,287
QuoteOriginally posted by Adam Quote
Thus, if you mounted something like a FA-645 55mm on a K-3, you'd get the same exact field of view as from any other K-mount 55mm lens
QuoteOriginally posted by mee Quote
The focal length stayed 55mm but the field of view changes. So how can someone say the field of view (that is, the framing) doesn't change -- it does change!
I may be mistaken, but I think you've misunderstood Adam. Your example changes the sensor size and keeps the lens the same (field of view changes here). His quote above is referring to changing between two 55mm lenses but keeping the sensor the same. Like comparing the field of view of a FA-645 55mm on a K-3 and the field of view of a SMC Pentax-DA* 55mm on a K-3, in this case the field of view is the same.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
aps-c, camera, crop, crop factor, factor, k3, lens, pentax help
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Confused about APS-C crop factor ripper2860 Troubleshooting and Beginner Help 21 06-01-2014 03:22 PM
FF or just less of a crop factor? K David Pentax Full Frame 14 11-08-2013 04:28 AM
How Far Is Pentax From Becoming A Major Player In APS-C? rbefly Photographic Industry and Professionals 32 10-30-2013 06:48 AM
How to shoot a wedding with APS-C and FF nick52 Photographic Technique 8 08-05-2013 05:14 PM
Whats the crop factor for a 6x7 lens on a APS-C K-mount camera? 2her0ck Troubleshooting and Beginner Help 8 06-16-2013 11:45 AM



All times are GMT -7. The time now is 05:59 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top