Which lens (1 Viewer)

[Adey Baker]

As you can see in my first post, that was my initial reaction... but Mike's prompt reminded me that this is a phenomena with this lens and the EOS mount (not sure about Minolta). In fact there are a few oddities about 170-500, the actual focal length for one.

I know the 50-500 was tested by a U.K. photo mag when it was released as being only 450-460mm at the long end, yet I have seen claims for the 170-500 as being up to 550mm on an EOS mount

regards,
Andy

I've just checked the test report in 'AP' on the latest Tamron 200-500mm F5-F6.3 and that also showed as F5.6 from about 300mm all the way down to 500mm on the tester's EOS5 - I'm sure it's the lens makers' way of ensuring auto focus on Canon bodies.

I'm not sure how they measure focal-length nowadays, with internal-focussing lenses constantly changing as you focus. It used to be measured at the infinity setting but I don't think many people use a long telephoto at the farthest focus setting, anyway. I know that Leica used to give a focal-length at the distance they expected them to be used at in practical situations, so that the 280mm and 560mm lenses were near enough the same as other makers' 300mm and 600mm lenses.

If some testers measure it at infinity and others use a different method (just guessing here - perhaps measuring the distance when a lens just fills the frame with a test chart, say) then they're bound to come up with different figures for the same lens.

As an aside, I've got two Sigma 400mm Apo lenses -one in OM fit and the other in EOS fit - and the tests in 'AP' which were both done by the same tester (the late Stewart Bell) showed quite marked differences in focal-length.

At infinity the older model was just over the marked 400mm but, in order to accomodate the closer focussing on the newer model the focal-length at infinity was almost 487mm (it drops to about 265mm at 1.6 metres!)

However, looking through the viewfinder - and even allowing for the slightly smaller viewfinder coverage of the Canon body - I can't see any noticeable difference with the lenses set at infinity! I've even held the OM-fit one next to the Canon body just to double-check and they're still both near enough identical so I'm not sure where that supposed 20% difference has gone to!

But as Mike says, so long as it does the job it doesn't really matter!

 

[Kevin Bates]

One thing to remember is that the focal length on a Digital Camera like the 20D is multiplied by 1.6 so 500mm is 800mm. I have had the focal length of my Sigma checked and its 495mm, my Canon 100-400 is 380 at the telephoto end.

 

[madmike]

More on lenses!

Hi Kevin,
I thought the factor was 1.5! whatever, this factor could be the cause of some variation as it may not be exact.
Re the Sigma lens F stop (which is getting away from the original question) I put the lens on my old Eos Film camera and it showed F5.6 so I guess it must go to that!
Does anyone know the 'exact' factor for these digital cameras?


Rgds,
madmike

 

[Andy Bright]

Hi Kevin,
I thought the factor was 1.5! whatever, this factor could be the cause of some variation as it may not be exact.
Rgds,
madmike

It varies according to sensor size, smaller = bigger crop factor... Nikon's give a crop factor of 1.5x, the Canons (except the Id series, which go up to full frame, therefore 0 cropping) are 1.6x.

regards,
Andy

 

[Kevin Bates]

You beat me to it Andy

The best way to test the focal length is on a 1.1 ratio camera which is what was used for my lenses.

 

[Adey Baker]

You beat me to it Andy

The best way to test the focal length is on a 1.1 ratio camera which is what was used for my lenses.

Just out of curiosity, how was the focal-length measured?

 

[Kevin Bates]

It was done by a friend that works in the optical business, they used marker boards to check the focal length and focus, similar to what you see in the test shots in magazines.

 

[Adey Baker]

It was done by a friend that works in the optical business, they used marker boards to check the focal length and focus, similar to what you see in the test shots in magazines.

Thanks, Kevin

Basically then, the focal-length quoted is only for the distance at which they test it

 

[paul goode]

Hi I am a new member,I am looking to buy as suitable for bird watching, I think I have narrowed it down to a 70-200 f2.8 with a x2 converter, or 50-500 mm or 80-400mm sigma lens, which one would be the most suitable, bearing in mind it will be used with a D70, and in Ireland were light levels aren't the highest.
Any help,
Brian

Hi Brian,

I'm coming into this thread rather late and you may well have made a purchase.
I've been using a 70-200 with a 2x converter for a while and got good results but I have just bought a 50-500 and am delighted with the results. Although it is early days with the lens the pictures appear sharper and the autofocus is quicker than the smaller lens with the converter. Used on its own the 70-200 is a great lens but lacks the reach for bird photography. The only downside of the 50-500 is that it's heavy and I have found it's only really useable on a heavy tripod. If you haven't made your mind up I hope this helps. By the way, most of the pictures in my gallery were taken around Birmingham where the light probably isn't much better than Ireland.

Regards

Paul

 

[guaycc]

Don't know about about Minolta, but the Canon body has a significantly shorter distance between the lens and the focal plane. This could have accounted for the difference in f/ stop (closer = brighter). Which is why both film and digital Canons register f5.6

The crop factor doesn't make a difference to the focal length - a 500mm is still a 500mm. It's all marketing hype that says we have gained extra mm's. All the lens elements and the focal plane remained in exactly the same position. Hence image magnification remained the same. It hasn't brought the bird closer to you. Only 2 things changed:
1 The CCD receiving the image is smaller (hence crop factor). It's like shooting a slide then masking the edges by approx 33% for 1.5 crop factor.
2. We have to buy new wide angle lenses for our DSLRs.