Nagging Focal distance doubts

[tsystem]

Ok, I've got a picture explaining my question. Here it is:

http://www.sgshirts.com/photos/misc/question.jpg
Due to the size of the picture, I'm posting a link instead of the image itself. Please view first.

Both pics are taken at 50mm focal distance, lense indication and exif confirms the focal distances of both lenses used. Distance from the clock to the sensor plane is the same at 45cm for both lenses used.

Question is: Why is one pic a bigger crop then the other? Why is the clock in the 2nd pic bigger than the first one?




This may cause some serious laughter for some, but its been bothering me, and would like some serious answers.

Opinions and Answers greatly appreciated, and helps and links to articles which will answer my query are also greatly appreciated.

Thank you in advance for your time.

 

[ckiang]

Very simple. The actual focal length of lenses are usually not as indicated, e.g. 50mm may actually be 55mm or 45mm. This is especially true for zoom lenses, where the variances may be bigger.

Regards
CK

 

[tsystem]

Originally posted by ckiang
Very simple. The actual focal length of lenses are usually not as indicated, e.g. 50mm may actually be 55mm or 45mm. This is especially true for zoom lenses, where the variances may be bigger.

Regards
CK

Thanks for your reply CK.

I would also like to know if the Exif data is also inaccurate, coz i've actually compared the EXIF focal distance info. And whether the D used in the camera to judge distance and flash distance uses the same focal distance info as the one indicated in the EXIF.

 

[ckiang]

Originally posted by tsystem
Thanks for your reply CK.

I would also like to know if the Exif data is also inaccurate, coz i've actually compared the EXIF focal distance info. And whether the D used in the camera to judge distance and flash distance uses the same focal distance info as the one indicated in the EXIF.

EXIF info depends on what the lens tells the camera. So if that 50mm lens is actually a 55 but tells the camera it's a 50, you're in the position you're in. The more accurate way to do this kind of measurement is by angle of view.

Yes, the D thing (for Nikon) is used in exposure calculations, especially flash exposure.

Regards
CK

 

[tsystem]

Thanks Ckiang.... I really appreciate your reply.

Took the two pics actually to compare the sharpness (perhaps?) between the two lenses.... to see if I can tell theres a difference 100% blown up. Ended up seeing two pics of different CROPS... was all jumpy.... :bsmilie:




Any other opinions or explainations available?

 

[YSLee]

CK's got it down quite well.

 

[rncw]

Originally posted by tsystem
Thanks Ckiang.... I really appreciate your reply.

Took the two pics actually to compare the sharpness (perhaps?) between the two lenses.... to see if I can tell theres a difference 100% blown up. Ended up seeing two pics of different CROPS... was all jumpy.... :bsmilie:




Any other opinions or explainations available?

so which lens gave you a sharper image? like to know as i'm oso eyeing the 28-105.

 

[ckiang]

Originally posted by rncw
so which lens gave you a sharper image? like to know as i'm oso eyeing the 28-105.

50mm f/1.8 of coz. My experience with the 28-105 is that like most zoom lenses, it's not sharp wide open. Need at least f/5.6 to get sharp enough pics. Especially at the 28mm end.

Regards
CK

 

[tsystem]

Originally posted by rncw
so which lens gave you a sharper image? like to know as i'm oso eyeing the 28-105.

As you can see from the diagram, my pics were taken both using f/8. Both pics were of the same sharpness to me at f/8. I've not tried f/3.5 for both lenses yet.

 

[ckiang]

Originally posted by tsystem
As you can see from the diagram, my pics were taken both using f/8. Both pics were of the same sharpness to me at f/8. I've not tried f/3.5 for both lenses yet.

f/8 is the optimal aperture for most lenses.

Regards
CK