Just wondering for wondering sake (tcons f/stop)

[timmyquest]

When i put a 2x on a 2.8 lens the f stop is doubled to 5.6 but what i'm wondering if the lens is actually stopped down or if that is a restriction within the converter?

[Ballen Photo]

When i put a 2x on a 2.8 lens the f stop is doubled to 5.6 but what i'm wondering if the lens is actually stopped down or if that is a restriction within the converter?

Timmy, Lets say you have a 100mm f/2 lens, and you mount a 2X T-con on it. You now have an effective 200mm lens, but the front element is still the same size it was before. You have now physically lengthened the lens without adjusting the front element size to keep the mathamatical equasion equaling f/2.
I dont have the actual formula in front of me right now to help better explain this, but I hope this helps you to understand how it works. :shock:
-Bruce

[AzzKicker]

I also had this question. SO for example.


F/2.8 lenses are usually SOFT at 2.8, but with the addition of a TC say 1.4x it goes to like 5.6, does that make the lens sharper? Or it still has the F/2.8 softness

[lcoleman]

As I understand it, the f/stop is a simple fraction of the length of the lens and the size of the aperture. So 100mm lens with an aperture opening of 35.7mm is 100/35.7 = 2.8. By adding a teleconverter that doubles the focal length of the lens, the same size aperture is now a smaller fraction of the effective length of the lens. So our example of a 100mm lens with a 2x converter goes to a 200mm lens with the same 35.7mm opening. The ratio now looks like this 200/35.7 = 5.6. This ratio is what makes fast lenses so big and expensive.

[Ballen Photo]

I also had this question. SO for example.


F/2.8 lenses are usually SOFT at 2.8, but with the addition of a TC say 1.4x it goes to like 5.6, does that make the lens sharper? Or it still has the F/2.8 softness

At BEST the sharpness will stay the same, as you're still using the same lens. Your DOF will be shallower though, just like using a telephoto that is equivalent to the total length of T-con plus lens.
.......Bruce

[Tom W]

I also had this question. SO for example.


F/2.8 lenses are usually SOFT at 2.8, but with the addition of a TC say 1.4x it goes to like 5.6, does that make the lens sharper? Or it still has the F/2.8 softness

No, the lens will be at least as soft as it is at f/2.8.

The f-stop is actually a mathematical relationship of focal length divided by the f-stop number. For example, a 50 mm lens at f/2 would have a 25 mm aperture diameter (50/2=25). Now if you took a 2x teleconverter and attached it to that 50 mm lens (assuming you could actually do so), you'd have a 100 mm focal length lens. However, the aperture diameter would not physically change (still 25 mm), so you'd now have an f/4 lens (100mm focal ength/25mm aperture=4) or (100mm/f4=25mm aperture).

So, the physical and optical characteristics of the main lens would not change, but you would simply be adding in the effects of the teleconverter. If the TC is real clean, the lens sharpness will change very little, but if the TC isn't so nice, it will add to the softness, plus it will take away the f-stop just as the math says it should.

[Ballen Photo]

The f-stop is actually a mathematical relationship of focal length divided by the f-stop number. For example, a 50 mm lens at f/2 would have a 25 mm aperture diameter (50/2=25). Now if you took a 2x teleconverter and attached it to that 50 mm lens (assuming you could actually do so), you'd have a 100 mm focal length lens. However, the aperture diameter would not physically change (still 25 mm), so you'd now have an f/4 lens (100mm focal ength/25mm aperture=4) or (100mm/f4=25mm aperture).


Thanks Tom. :D
-Bruce

[timmyquest]

I also had this question. SO for example.


F/2.8 lenses are usually SOFT at 2.8, but with the addition of a TC say 1.4x it goes to like 5.6, does that make the lens sharper? Or it still has the F/2.8 softness

No, the lens will be at least as soft as it is at f/2.8.

The f-stop is actually a mathematical relationship of focal length divided by the f-stop number. For example, a 50 mm lens at f/2 would have a 25 mm aperture diameter (50/2=25). Now if you took a 2x teleconverter and attached it to that 50 mm lens (assuming you could actually do so), you'd have a 100 mm focal length lens. However, the aperture diameter would not physically change (still 25 mm), so you'd now have an f/4 lens (100mm focal ength/25mm aperture=4) or (100mm/f4=25mm aperture).

So, the physical and optical characteristics of the main lens would not change, but you would simply be adding in the effects of the teleconverter. If the TC is real clean, the lens sharpness will change very little, but if the TC isn't so nice, it will add to the softness, plus it will take away the f-stop just as the math says it should.

That clears a bit up.

[ron chappel]

An f2.8 lens with a 2x TC becomes an f5.6 lens.

But NO,it isn't stopped down-the main lens is still at f2.8 (at it's softest aperture!) :cry:

[drisley]

If your lens is sharp to start with, then a 1.4x tcon will show almost no quality loss.
This (http://www.fotop.net/sharpnsmart/zoo/CRW_2546) was taken with my 135F2L and 1.4x tcon wide open ( f2.8 )