If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
Somebody PLEASE explaing this to me... - Focal Length
Bill W wrote:
On Thu, 27 Aug 2015 23:02:25 -0400, nospam wrote: In article , Bill W wrote: I am going crazy with this. Two lenses, side by side. A Pentax DA L 70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's designed for APS-C sensors, and the same with the DC on the Sigma lens. Why do they have nothing close to the same field of view & magnification at the same focal length? To get the same photo with both lenses, I need to set the Pentax to 170mm, and the Sigma to 250mm. When I do that, and then look at the two identical photos in LR, the exif data shows the Pentax set at 170, with a 255mm 35mm equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which is correct, I guess, but the Pentax has much greater magnification at *equal* focal lengths. that's called focus breathing, where the actual focal length changes as you focus on closer subjects. lots of zoom lenses do it, especially superzooms like the sigma. try comparing with it focused at infinity. This was indeed the issue. The field of view at 250mm is now nearly identical when focused at, or near, infinity. Just make sure you understand that focus breathing is a non-issue. It just doesn't matter. -- Floyd L. Davidson http://www.apaflo.com/ Ukpeagvik (Barrow, Alaska) |
#22
|
|||
|
|||
Somebody PLEASE explaing this to me... - Focal Length
|
#23
|
|||
|
|||
Somebody PLEASE explaing this to me... - Focal Length
On 8/28/2015 10:58 PM, Floyd L. Davidson wrote:
PeterN wrote: On 8/28/2015 10:45 AM, PeterN wrote: On 8/27/2015 10:59 PM, Floyd L. Davidson wrote: Bill W wrote: I am going crazy with this. Two lenses, side by side. A Pentax DA L 70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's designed for APS-C sensors, and the same with the DC on the Sigma lens. Why do they have nothing close to the same field of view & magnification at the same focal length? To get the same photo with both lenses, I need to set the Pentax to 170mm, and the Sigma to 250mm. When I do that, and then look at the two identical photos in LR, the exif data shows the Pentax set at 170, with a 255mm 35mm equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which is correct, I guess, but the Pentax has much greater magnification at *equal* focal lengths. Can someone explain this to me? I am thinking of buying the Sigma 18-300 - also a DC lens, but it now appears that the Pentax would still have greater magnification (375mm equivalent). How can one compare the actual magnification of different lenses on paper? It's obvious in this case that one needs to multiply the Pentax focal length by 1.5, which is expected, but not the Sigma. How can I know that going forward, and looking at other lenses? Which cameras are you using? Note that at the same focusing distance and the same actual focal length, the "magnification" will be identical. Just be aware that magnification means the comparison of the projected image to the actualy object's size. It has nothing to do with the size of the image you get! You might post a couple images, one from each camera/lens, to a webpage or to alt.binaries.photos.original. If we could download the image and look at Exif data it would help. Your theory is spot on. However, some lenses have what I would call optimistically labeled focal lengths, especially at closer distances. My Nikon old 18-200 is an example. At close distances, I would estimate the longer end as being closer to 160 than 200. I shold clarify, that issue exists with zoom lenses. I have not noticed it with prime lenses. Zoom lenses have no lock on focus breathing. But don't miss the point that the focal length at the minimum focus distances make no difference at all! Who cares what it is! What you want is at least some minimum magnification. If the lens provides that... The fact is that focus breathing is what allows the lens to focus that close with fewer aberrations. If the design target required the focal length to remain longer, the lens simply would not be able to get the same quality at the same magnification! Ah! -- PeterN |
#24
|
|||
|
|||
Somebody PLEASE explaing this to me... - Focal Length
In article ,
RichA wrote: I am going crazy with this. Two lenses, side by side. A Pentax DA L 70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's designed for APS-C sensors, and the same with the DC on the Sigma lens. Why do they have nothing close to the same field of view & magnification at the same focal length? To get the same photo with both lenses, I need to set the Pentax to 170mm, and the Sigma to 250mm. When I do that, and then look at the two identical photos in LR, the exif data shows the Pentax set at 170, with a 255mm 35mm equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which is correct, I guess, but the Pentax has much greater magnification at *equal* focal lengths. that's called focus breathing, where the actual focal length changes as you focus on closer subjects. A horrible SCAM perpetrated by them, Nikon and others. The Nikon 70-200mm at 200mm is actually close to 115mm when used close-up. What a con. it's not a scam and it doesn't actually matter. most lenses, including whatever you use, have focus breathing. |
#25
|
|||
|
|||
Somebody PLEASE explaing this to me... - Focal Length
On Sat, 29 Aug 2015 12:17:57 -0700 (PDT), RichA
wrote: On Thursday, 27 August 2015 23:02:29 UTC-4, nospam wrote: In article , Bill W wrote: I am going crazy with this. Two lenses, side by side. A Pentax DA L 70-300 lens, and Sigma 18-250 DC lens. The DA in Pentax means it's designed for APS-C sensors, and the same with the DC on the Sigma lens. Why do they have nothing close to the same field of view & magnification at the same focal length? To get the same photo with both lenses, I need to set the Pentax to 170mm, and the Sigma to 250mm. When I do that, and then look at the two identical photos in LR, the exif data shows the Pentax set at 170, with a 255mm 35mm equivalent, and the Sigma at 250, with a 375mm 35mm equivalent, which is correct, I guess, but the Pentax has much greater magnification at *equal* focal lengths. that's called focus breathing, where the actual focal length changes as you focus on closer subjects. A horrible SCAM perpetrated by them, Nikon and others. The Nikon 70-200mm at 200mm is actually close to 115mm when used close-up. What a con. Oh how you must have suffered! I find the cure is to concentrate on the image that the lens is producing and to stop worrying about the numbers that might be associated with it. Just look at the picture. -- Regards, Eric Stevens |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
The length of the focal length? | [email protected] | Digital SLR Cameras | 12 | October 5th 07 12:02 PM |
Fixed focal length DX? | Beemer | Digital Photography | 5 | November 16th 06 08:03 PM |
Is there a formula to convert digital lens focal length to 35mm focal length ? | narke | 35mm Photo Equipment | 5 | March 1st 05 01:31 AM |
focal length calculation | TS | Other Photographic Equipment | 2 | August 7th 04 08:33 PM |
Best Focal Length for Portraits. | [email protected] | 35mm Photo Equipment | 47 | July 28th 04 07:43 PM |