Cross Breeds

With a 650-1300 mm Telescopic Zoom Lens for Sony Alpha 100.Will I be able to use a lower ISO number?

I like to take pictures of race horses and other action photos but on dull days I have to increase the ISO number so much that the result is very grainy.I have the 75mm/300mm and the 18mm/70mm zoomlenses If I invested in a 650-1300 mm Telescopic Zoom Lens for my Sony Alpha 100 would I be able to lower the ISO number?

Public Comments

1. It's likely that you'll be increasing your shutter speed to compensate for camera shake. This will mean an even higher ISO number. You need a faster lens, not a longer one.
2. No, You will be raising your ISO even higher. I don't know which lens you are talking about but a zoom lens like that is probably at least an F8. The higher the aperture or F number the less light gets into the camera. Less light equals slower shutter speeds. To compensate you would have to increase your ISO. Also there is no way you could hand hold a lens like that. It would have to be on a tripod. The other thing is that lens is almost certainly optically terrible. Long telephoto lenses cost thousands of dollars if they are any good. If the lens is the Rokinon stay away. It is a worthless piece of crap. There is no auto focus and it is a variable aperture. Meaning that at 650mm it is F8 which is pretty slow. By 1300mm it is F16 which is really slow. It would be of no use whatsoever unless it was extremely bright out. Even then optically speaking its terrible. There is a reason why real lenses have this size cost more that 10 times that amount.
3. In general, the larger the magnification of the lens, the more light you need so the ISO need would increase. However, every lens has a minimum F-stop rating that tells you the largest aperture possible on that lens. Lenses with f2.8 need far less light (at that setting) than lenses which can only open to f4 or f5.6. If you are currently getting enough magnification out of your current lenses I would look at investing in faster glass (the smaller f-stop rating) rather than more magnification.
4. Absolutely not. The lenses I looked at with the 650-1300mm range are f8-16 so when you zoom to 1300mm you'll lose two full stops. Due to its very low price you have to figure that the overall quality is poor. I really doubt that its stated f-stop at 650mm is really f8. It may be accurate but the overall light transmission is probably slower. If I were you I'd check the lens against a 50mm prime at f8 and see if there's any difference in the meter readings. Since this lens will not auto focus with your camera and probably doesn't couple to the meter you'll have to shoot in Manual and use stop-down metering. Another consideration with ultra-long lenses is the amount of haze/pollution in the air. The slightest amount will be greatly magnified (compressed) by such a long lens and your image quality will suffer. A UV/Haze filter might help some but don't expect miracles. In my opinion this lens is a waste of money.