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420mm vs 500mm field of view (1 Viewer)

rhanief

Member
Hello all,

I already have a Sigma 100-300mm f/4. I am thinking to buy a 1.4x TC, which will extend the reach to 420mm.

I wonder how much is the difference between 420mm and 500mm? It would be great if you can show me a direct image comparison from the same scene.

If I shoot at 420mm on D200 (10.2 MP), how many pixels should be cropped to get the same field of view as 500mm?
 
~16% increase in FOV

One way is to use Bob Atkins' calculator at--
http://www.bobatkins.com/photography/technical/field_of_view.html
(About middle of page)

Mothman's charts show the field of view for focal lengths from 17 to 600mm at distances from 5 to 90 feet.

5 - 30 feet:
http://www.pbase.com/mothman13/image/74511515

35 - 60 feet:
http://www.pbase.com/mothman13/image/74511517

65 - 90 feet:
http://www.pbase.com/mothman13/image/92899167

Mothman's tutorial image viewer--
On this page, you can select one of four focal lengths (300mm, 400mm, 500mm, 600mm), one of 4 distances (100 feet, 150 feet, 200 feet, 300 feet) and one of three birds (12" Blue Jay, 24" Mallard and an Osprey with 65" wingspan). Click the button and you'll see a picture showing what the full photo would look like with your selections (scaled to 800 x 533 to show on the page). Switch one or more parameters and you can make direct comparisons.

http://www.texasmothman.com/photography-tutorials/birding/birding.asp


Generally, cropping FOV is ~linear--
420-mm/500-mm = ~0.84
0.84 x 500-mm = 420-mm

~16% crop of 420-mm FOV to yield ~500-mm FOV

In my experience, > ~30% FOV cropping is when one starts to take a hit on IQ. It's almost always better to have the longer reach than to achieve the same FOV by cropping for comparable IQ lenses-- unless one is starting with a prime and comparing to a long end of a zoom.

Here's a Bigma image at 320-mm (480-mm EQ)
DSC_2958Medium.jpg


at 420-mm(630-mm EQ)
DSC_2959Medium.jpg


at 500-mm (750-mm Eq)
DSC_2960Medium.jpg


Keep in mind that adding a TC will always have an impact on IQ; less for a 1.4X than a 1.7X TC; less if on a great prime than on a zoom; Generally, a 1.7X TC is about the upper limit one would want to attempt -- IQ will ALWAYS be best without a TC
 

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Last edited:
Hi Dave, thanks for a very comprehensive info!

I cropped your sample image at 420mm to match the 500 mm. The image size was reduced to 661x439 from 800x532. The ratio is ~0.82 in both horizontal and vertical axis, very close to the theoretical calculation. The area would be reduced to 0.82 x 0.82 ~ 0.67.

Roughly, I will get 6.8MP instead of 10.2 MP from my D200. Yes, this is a huge lost, because usually I need to crop more. But I still can live with it because my old D70s also produce 6MP.

I decide to buy sigma APO TC 1.4x, because tamron 200-500 or bigma is still beyond my current budget. At least I can use the combo until I can afford another lens :).
 
What you are failing to realise is that each time you crop and increase apparent image size you also increase the size of the individual pixels, this makes the image appear grainier and therefore less sharp! There is no real substitute for real image size as opposed to cropped and increased image size.

nirofo.
 
What you are failing to realise is that each time you crop and increase apparent image size you also increase the size of the individual pixels, this makes the image appear grainier and therefore less sharp! There is no real substitute for real image size as opposed to cropped and increased image size.

nirofo.
Yes, thanks for remind me.
So far, the trade off is still acceptable and I'm happy with the combo ;).
 
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