Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-07-2013, 03:15 PM   #1
Senior Member




Join Date: Aug 2013
Location: Schwyz
Posts: 135
Mis-Understanding "Crop-Factor"

Hello

I did some testing with lenses i use.

First I am very happy the the old Analog SMC FA 28-200 Lens. A couple of weeks ago I bought also the newest SMC Pentax DA 50-200mm ED WR lens.

So I thought the old 28-200 used in an APS-C envoirement would convert to an 42-300mm lens (related to the 1.5 cropfactor of the APS-C Chip. Now I compared some pictures and i think the cropfractor should be 0.6875

I took 1 picture with my good old 28-200 at 200mm and another with the new Pentax 50-200

When I use the Pentax DA-L 50-200mm I have the same perspective if I reduce the zoom to 137.5mm?

What is the problem. I would suggest it should be reversed. Can someone help me?

Regards
Gérard

Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
10-07-2013, 03:24 PM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 49,167
QuoteOriginally posted by Gerard_Dirks Quote
Hello

I did some testing with lenses i use.

First I am very happy the the old Analog SMC FA 28-200 Lens. A couple of weeks ago I bought also the newest SMC Pentax DA 50-200mm ED WR lens.

So I thought the old 28-200 used in an APS-C envoirement would convert to an 42-300mm lens (related to the 1.5 cropfactor of the APS-C Chip. Now I compared some pictures and i think the cropfractor should be 0.6875

I took 1 picture with my good old 28-200 at 200mm and another with the new Pentax 50-200

When I use the Pentax DA-L 50-200mm I have the same perspective if I reduce the zoom to 137.5mm?

What is the problem. I would suggest it should be reversed. Can someone help me?

Regards
Gérard
You should ignore the whole crop factor concept completely. If the lens says 200mm, you'll get the same field of view whether it's a DA or a FA lens.

Crop factor essentially just means that to get the same field of view as a 200mm lens on a film camera, all you need is a 200/1.5 so about 133mm on digital camera. But two different 200mm lenses will always give you (approximately, due to inaccuracies in markings etc) the same field of view.

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



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

10-07-2013, 10:55 PM   #3
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,854
As Adam says, crop factor is used when comparing formats, I.e. full frame vs APS-C for example. It is not used when comparing lenses within a format.

Focal length is focal length. That is all.
10-07-2013, 11:13 PM   #4
Senior Member




Join Date: Aug 2013
Location: Schwyz
Posts: 135
Original Poster
Thanks.

That means also that wenn I use my old Mirror 500mm lens on my K5 i only get the field-angle as with a new 333mm lens

So all the people who write that an old 200mm will be the same as an new 300mm are not correct. An old 200mm is the same as a new 133mm. Still very confusing.

Gérard

10-07-2013, 11:52 PM   #5
Senior Member




Join Date: Aug 2013
Location: Schwyz
Posts: 135
Original Poster
Hello

Does this mean when i use my old SMA FA 28-200mm on my K5, it would be the same result as using a new Pentax DA 18-135WR ?

Gérard
10-07-2013, 11:58 PM   #6
Veteran Member
SteveM's Avatar

Join Date: Jan 2009
Location: Vancouver Island, BC, Canada
Photos: Gallery
Posts: 3,294
QuoteOriginally posted by Gerard_Dirks Quote
Hello

Does this mean when i use my old SMA FA 28-200mm on my K5, it would be the same result as using a new Pentax DA 18-135WR ?

Gérard
No. The field of view will only change when changing the format of the camera. A full frame camera vs. a crop format camera.
10-08-2013, 12:26 AM   #7
Veteran Member
carrrlangas's Avatar

Join Date: Mar 2012
Location: Joensuu (Finland)
Photos: Gallery
Posts: 1,761
crop factor is an attempt to quickly tell how a lens will look compared to the known "full frame" format. Aka 35mm film, aka 135 format.

Thing is, for any given format (format is the dimensions of the sensor) there is the next convention:

Ultra wide angle lens = focal lenght shorter than short side of the sensor
wide angle lens = focal length shorter than long side of the sensor
normal lens = focal length arround diagonal of the sensor
normal telephoto lens = focal length arround double of the diagonal of the sensor
telephoto = focal length longer than double of the diagonal of the sensor

also, focal length is a property of the lens. it won´t change. What changes is the format.. With the above rule and knowing that pentax DSLRs use APS-C sensors which are 15,7mm x 23,5mm
10-08-2013, 06:53 AM   #8
Loyal Site Supporter
Loyal Site Supporter
JimJohnson's Avatar

Join Date: Jan 2012
Location: Summer:Lake Superior - Michigan Winter:Texas Hill Country
Photos: Gallery
Posts: 2,356
Think of it this way @Gerard_Dirks - take a photo with a film camera and print it. Now trim away 25% of the photo from all sides leaving you 75% of the original photo. Now take your cropped photo over to a copier and enlarge it back to the same physical size as the original pre-cropped photo (150% enlargement).

That is EXACTLY the relationship between the 35mm film camera and your APS-C digtial camera. The same lens sees the exact same thing on both cameras, only the digital camera uses only the central 75% of the image because the sensor is smaller than a frame of 35mm film.

That is what people are trying to tell you - focal length is focal length. The image is the same regardless which camera format is used for a given focal length. The only difference is the digital camera is only using 75% of the image.

10-08-2013, 08:57 AM   #9
Veteran Member




Join Date: Sep 2006
Location: WV
Posts: 1,495
The focal length is a physical property of a lens. It never changes. Crop factor is a function of sensor (or film) size and has nothing to do with the physical properties of a lens.
10-08-2013, 09:42 AM   #10
Site Supporter
Site Supporter
forensicscientist's Avatar

Join Date: Jun 2008
Location: DFW Texas and Michigan - I commute :)
Posts: 958
1/0.6875 = 1.4545 or about 1.5
10-08-2013, 11:49 AM   #11
Veteran Member
maxfield_photo's Avatar

Join Date: Mar 2011
Location: Cincinnati, OH
Posts: 1,214
It's the camera that applies the crop factor, the lens just projects whatever image it's going to project. Let's look at it two different ways: first, "equivalent" focal lengths.

Let's use a "normal" prime for comparison (and by the way, "normal" refers to the amount of perspective vs. what one would observe with the naked eye, not the field of view). On 35mm film, or a full frame digital camera 43mm is the "normal" focal length, although you will hear lenses as long as 55mm referred to as "normal". You can calculate it by taking the square root of the diagonal of the image area (24^2+36^2=5625, sqrt(5625)=43). The FA 43 limited was designed to be a "true normal" lens, but remember, it was designed for film. Put the FA 43 on a cropped-sensor body, and it becomes a short telephoto lens. The focal length is still 43mm, but now it gives the field of view equivalent to what a ~65mm lens would on a 35mm film camera. Now this is the part that's hard for some folks to grasp. Although the crop of the camera does not change the contents of the image at the center of the frame (i.e. the perspective), it does cause the information at the edge of the frame to be discarded. In order to regain that information, one must move the camera back, the act of which, in practice, changes the perspective of the lens. Perspective is 100%, completely and totally a function of camera position relative to the subject within a given format. To capture roughly the same image (i.e. a normal perspective) on a cropped-sensor camera, you would need a lens that is 33% shorter. So a 28mm lens functions as a normal lens on a cropped body. This would allow you to keep the camera in the same position.

Now let's look at it another way. Let's take the same focal length from three different formats and look at it on the same body. We'll use the DA* 55mm, the K series 55mm, and the 645 D FA 55mm. The lenses were respectively designed for cropped-sensor digital, 35mm film, and cropped-sensor medium format. Put all three lenses on a cropped-sensor camera like the new K-3 and you will get the exact same image (with the possible exception of less vignetting and more corner sharpness from the 645 lens), although one lens was designed as a portrait length, one as a "long normal", and one as a "true normal" for their respective formats. Now let's put all three lenses on a larger format, like the 645D. With the DA* 55 and the K 55, you are likely to get heavy, if not total vignetting in the corners, but still the same image from all three lenses. The 645 lens is a larger diameter and projects a larger image circle, but a 55mm lens is a 55mm lens.

So the crop factor is completely a function of the camera, and lens focal lengths are absolute, not relative measurement. Terms like "normal", "telephoto", "portrait", and "wide" however are relative and their definitions change with the format. Hope that helps, though others have done a good job of explaining too. Sometimes equivalency is easier to understand in the terms that make the most sense to the individual.

Last edited by maxfield_photo; 10-09-2013 at 12:35 AM.
10-08-2013, 12:24 PM   #12
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,854
In re reading the OP's post perhaps we have all taken it incorrectly. Perhaps the 28-200 sufferes considerable focal length shortening when focusing at minimum distance, and this is leading him to confuse a property of high zoom ratio lenses with crop factor. The 55-200 is a more conservative 3.9:1 zoom ratio and will suffer less focal length shortening than the 6.6:1 zoom.

Just a thought
10-08-2013, 01:43 PM   #13
Site Supporter
Site Supporter




Join Date: Mar 2009
Location: Gladys, Virginia
Photos: Gallery
Posts: 21,084
So, there are two issues. First of all, your focal length is your focal length. A 200mm lens on your APS-C camera will look like all other 200 mm lenses -- FA *200, DA *200, DA 50-200 all should frame exactly the same on your camera. That said, if you would mount those lenses on a full frame camera, say a K1000, you would see a wider field of view than you do on your APS-C camera (assuming that the lens covers the larger sensor/film). So, the crop factor just indicates the relationship between an APS-C sensor, which sees less of the image, and a full frame sensor, which sees more of the image. Based on the crop factor, mounting a 200mm lens on a APS-C camera should give you a similar field of view to a 300mm lens mounted on a full frame camera.

The other thing that is different is what Lowell mentions, which is that when you focus on objects closer than infinity, internally focusing lenses will have a shorter focal length than their focal length at infinity. So, a DA 55-300 lens will only be at 300mm when focused on an object at infinity, but if you focus at its minimum focus distance, the focal length is more like 170mm.
10-10-2013, 11:11 AM   #14
Veteran Member
Andi Lo's Avatar

Join Date: Jul 2008
Location: Halifax, Nova Scotia
Posts: 2,925
If you dont have / not using a film camera, just forget that crop factor exists. It's irrelevant for you.

As someone mentioned above, your old FA lens might have FL shortening at close focus distances, which explains the strangeness that you're seeing.
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!
50-200mm, aps-c, lens, pentax, pentax help, smc
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Optical differences between Pentax "K", "M", and "A" lenses 6BQ5 Pentax SLR Lens Discussion 31 01-10-2014 01:02 PM
Definition of "100% Crop" John279 Digital Processing, Software, and Printing 12 11-15-2012 04:21 PM
Just Read "Understanding Exposure" - Meter Question hockmasm Pentax DSLR Discussion 7 05-05-2010 05:44 PM
Price factor aside, what's the best "fast 50"? noahpurdy Pentax SLR Lens Discussion 22 02-01-2010 09:44 AM
Does the "crop factor" just crop? jboyde Troubleshooting and Beginner Help 8 01-03-2010 08:18 AM



All times are GMT -7. The time now is 09:19 AM. | See also: NikonForums.com, CanonForums.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