Lens selection primer.

[awboater]

Some of you may recall I always talk about lens focal lengths being a 2x relationship. I ran across a cool Nikon Lens Simulator that might help illustrate this point, and perhaps give you a good idea of what to expect when you are shopping around for a new lens.

 

http://imaging.nikon.com/lineup/lens/simulator/index.htm

 

While this simulator is tailored for Nikon products, it will work in almost any other situation - with the caveat I illustrate later.

 

I have always said that it takes a 2x change in focal length for any significant difference. 2x means either doubling the focal length or halving it, depending on if you are ascending or descending in length. Going from 100mm to 200mm is a 2x change, from 100mm to 50mm is a 1/2x change, and so on.

 

Try this on the simulator: compare 100mm and 200mm; see that there is a noticeable and significant difference. Now compare 500mm and 600mm; hmmm, hardly any difference at all.

 

Why; Both are 100mm differences?

 

Here is the reason. There is a 2x difference from 100mm to 200mm, but only a 1.25x difference between 500mm and 600mm. When comparing focal length differences, you cannot simply look at the numerical difference (which would be a linear relationship), you have to look at it's multiplication factor (which is a logarithmic relationship).

 

So apply that to a couple of popular lenses people use for cruising; a Tamron 18-270mm vs. Tamron 28-300mm. There have been many discussions on which is better.

 

Therefore, consider these lenses in the 2x world. The smaller lens will give you more range, as there is a fairly noticeable difference between 18 and 28mm, but the difference between 270 and 300mm is hardly noticeable.

 

Again, this is the 2x world working. Notwithstanding any image quality differences between the two lenses (distortion and such), the 18-270 would be a better lens I think, as it would provide more range. This is confirmed by Tamron's literature; the 28-300mm is has an 11x range, and the 18-270 has a 15x range.

 

So what does 15x mean? How can you tell...

 

The easy rule of thumb is to double the focal length in 2x increments starting with the lowest end, and hope you come up with something close at the high end (unless you want to get your calculator out).

 

So here goes:

 

18mm x 2 = 36mm; x 2 = 72mm; x 2 = 144mm; x 2 = 288mm.

 

OK, x2 happened almost 4 times. So 2 ^4 = 16 (2 x 2 x 2 x 2). So if the lens had been 288mm, it would have been 16x, but since at 270mm it just fell short - then 15x seems correct; without the use of a big brain or calculator.

 

So buying the longest lens you can afford is not necessarily the best solution if you already have a similar lens, or are comparing two lenses with slightly different ranges. We already discussed the 18-270 vs 28-300 but now compare another two popular sizes; 55mm-200mm vs. 55mm-300mm focal lengths. Is the difference between 200mm and 300mm even worth purchasing that lens? To me, if you already have a 200mm lens, it would be a marginal improvement.

 

Also, if you are looking at Nikon lenses, you can directly enter those. But again, if you are just looking at generic relationships between focal lengths, you can discount any Nikon products.

 

Pretty neat, eh?

 

One thing that brought this topic up in my mind was the other day, I was looking at the Sigma 120-400mm f/4.5~5.6 which I have an interest in. The store didn't have this lens, but they had the 150mm-500mm f/5~6.3 version (which I understand is the same lens but with different front glass). At any rate, he told me that the 500mm is capable of shooting birds that the 400mm is just not capable of. To me, that just didn't seem correct, and I even tried the 150-500mm at the 400mm and 500mm points - and it confirmed my belief that there was not a lot of difference. I am still preferring the 120-40mm, as it is a bit faster (but again not significantly so), but it is also a bit lighter and smaller - which is more significant. I think the store owner was just trying to push what he had in stock. I have not made that purchase, so I suppose it's a future thing.

 

Oh the caveat... if you select a full frame body (FX body) and compare FX and DX lenses, you will see a shift in the image. This is NOT an optical difference, it's just that with Nikon FX bodies, they automatically detect Nikon DX lenses, and automatically shift into DX mode.

 

If you used a non-Nikon DX lens on a Nikon FX body (assuming the body would not detect it was a DX lens), the image size would not change, but you would perhaps see some vignetting in the photo. There is a lot of mis-confusion in what actually changes in a cropped camera, and this simulator could add to it if you don't realize what is happening. Discussing crop factors is a topic onto it's own.

 

So the next time someone asks a question about which lens is better, you can answer it...

 

Notes:

 

In the simulator, DX means a 1.5x crop factor camera or lens (APS sized image). FX means a full 35mm size sensor or lens that allows for the full frame.

[awboater]

Anyway, try out the simulator.

[zackiedawg]

Much depends on how close to the ragged edge you are shooting, in your case of the longer focal lengths. While your points are completely valid, and it would be good for those who don't realize it to understand how big the difference can be on the wide end between to relatively close sounding focal lengths (15mm vs 20mm), and how small the relative difference at the long end (400mm vs 500mm), it must also be said that birders especially are sometimes pushing the very limits of resolution and croppability, and those relatively small differences DO make a huge impact. When shooting a small bird from 100 feet, with a 400mm lens you are going to be cropping huge amounts of the photo to get detail on the bird - often, you are just beyond the threshhold and feather detail is mushy and lost. Go to 500mm, and you're still cropping 50% of the photo away, but now you're just this side of the threshhold, and you've managed to eek out individual feather detail that gives the photo the punch needed. I've even compared a BETTER lens at 420mm to an average lens at 500mm of the very same bird, and the 500mm won out, with that small difference in reach making just enough difference in feather detail to get me the shot, whereas the 420mm was just not quite enough to make a print or sell.

 

Not countering your advice or point - it's well made and good reference for many. But I just wanted to point out as a birder that the relatively small gains in those long telephotos actually does make a creditable difference when you're right on the edge of detail retention.

[awboater]

I'll admit I am not a birder, so I will concede your point that there are probably some ragged edge special purposes out there.

 

But at any rate, I thought the simulator was pretty neat, and it kind of puts the differences in focal lengths in perspective.

[zackiedawg]

Indeed it does. And though I do have to make use of every bit of focal I can get at times, I have often tried to explain this principle to others when they are talking about upgrading their P&S ultrazoom from one that can reach 420mm to one that can reach 500mm equivalent - they seem to think this is going to result in a huge extra reach - this link will help show them to expect much smaller, incremental gains.

 

That said, with 500mm being my furthest reach, I find myself lusting after the big 600mm F4 prime that costs as much as a decent used car. :)

[Jade13]

Is the difference between 200mm and 300mm even worth purchasing that lens? To me, if you already have a 200mm lens, it would be a marginal improvement.

 

quote]

 

It would have made a huge difference in Antarctica, and why the professional photographers on-board could get those "flying" (over the water) penguins when others could not.

 

I thought the 300mm made a big differance in the Arctic as I could get some reasonable photographs of polar bears.

(A 400mm would just be way too heavy for me).

[Jade13]

Notes:

 

In the simulator, DX means a 1.5x crop factor camera or lens (APS sized image). FX means a full 35mm size sensor or lens that allows for the full frame.

 

My Nikon D60 uses DX. I would like to know how the lenses will work if I ever purchase an FX camera body. I have a few old manual lenses from my Nikon FM that I have been planning on selling for years. Perhaps I should hold onto them?

[zackiedawg]

When dealing with longer focal lengths, the difference gets to be less and less the longer you go. There is a greater difference between a 100mm and 200mm than between a 200mm and 300mm...the difference from 200mm to 300mm is still noticeable, and more than going from 300mm to 400mm. And so on. Each 100mm becomes a smaller percentage increase over the previous focal.

 

As for DX vs FX - You can buy DX or FX lenses for your DX...the DX lenses wouldn't work on the FX (or would, but would show vignetting and have to be either set on the camera to use less of the sensor, or cropped afterwards), because they are made with smaller optics designed to match the smaller APS-C sensor of the DX camera. The FX lenses when put on an APS-C body simply use a center portion of the lens - they 'crop' the view but should work completely normally, just with the 1.5x crop factor taken into account. In general, if you think a full frame body may be in your future, you are better off investing in full frame lenses now...they work perfectly on your DX body, AND will be fully compatible if you go full frame. Your old manual lenses are already 'full frame', so they can work on both DX and FX - so it might be worth keeping them if an upgrade is in your future.

[awboater]

Jade;

 

Most of my lenses are full-frame, as I might or might not go FX in the future. With the exception of wide angle lenses, I always try to buy full fame lenses... you know, for the future.

 

The only DX lenses I have are wide angle lenses, as the crop factor especially works against you on the wide angle end, so you need a shorter focal length lens for the equivalent DX depth of field. Since the DX crop factor only uses the center of the glass, the DX wide angle lenses are smaller in diameter than a FX lens would be.

 

In reality, that is all a DX lens is; it's essentially a cropped FX lens which matches the crop in the sensor. If you are not using the entire FX sensor, you don't need the entire FX glass area on the lens.

 

That means the DX lenses should be cheaper, and I suppose they are to a point.

[awboater]

When dealing with longer focal lengths, the difference gets to be less and less the longer you go. There is a greater difference between a 100mm and 200mm than between a 200mm and 300mm...the difference from 200mm to 300mm is still noticeable, and more than going from 300mm to 400mm. And so on. Each 100mm becomes a smaller percentage increase over the previous focal.

 

 

That is the point I was trying to make. And the reason I am sure, is that lenses magnification rate is not linear as you move up into longer focal lengths. For me, the significant spot where this does not matter is at the 2x point. The difference may be noticeable, but not necessarily significant for my needs; which is one reason why my 70-300mm usually sits at home.

 

And that is why I was looking at a 120-400mm as for me, 400mm is a significant enough change from one of my 200mm lenses to warrant purchase.

 

We each have our preference as to what is significant. For others, especially birders as you state, it's obviously less than 2x. So the simulator seems a neat tool to help you visualize the differences and determine that point.

[Habenero]

As a simple test to see what would happen in the future when I finally go to a full frame sensor, I simply put my 10mm to 20mm DX lens on one of my ancient film cameras and watched the edges for vignetting. I discovered that at about 13mm the frame was clear without noticeable vignetting, which eans that I could still use the lens for many shots!:D

[awboater]

Also, for at least Nikon (and I am sure Canon and others do it well), the full frame cameras will recognize whether or not the lens is a FX or DX and go into DX mode. Not sure if they recognize all of the 3rd party lenses or not though.

 

But since your idea of staying in FX mode until getting rid of the vignetting is going to give you a superior photo - that might be the best bet.