WHAT IS A “FAST LENS”?
Let’s explain a bit about the numbers on lenses so that perhaps you will see where we are going with this.
Looking at all lenses, there are three numbers on the front of the lens that tells us what kind of lens it really is:
- The first number you see on the lens (like pictured above) is the actual “strength” of the lens, or better yet: the description of what this lens can do as far as perception. If an 28mm is now the normal lens on cameras, then that lens should be our reference point. By the way, they make a 28mm lens, which would in today’s world, when you look through it look like the same as what your eye sees. Look at a subject, then look through the 28mm lens, and you should see no change in magnification or push back of the subject. If you double that number to a 56mm, you are bringing the image in twice as close. Everything bigger than 28mm is a telephoto lens. Everything smaller than 28mm is a wide angle lens. Looking through the 56mm lens, the person, who may be standing 10 feet away from you will now look like he’s only 5 feet away from you. And doubling the lens millimeter again to 112mm, will bring the subject that much closer again. Get the picture. If you go to a 14mm lens, now the person will look like they are 20 feet away. And so forth. The interesting thing about changing lenses, is the amount of area you get or don’t get depending on the lens. So a 14mm lens will show double the amount of area in the picture. And the same goes with the telephoto. Go to a 56 mm lens, and you only have half the area in the picture compared to the 28mm.
The second number signifies the “speed” of the lens.
- The second number you see on the front of the lens is the “speed” of the lens, or how much light this lens will let through it. The smaller the number, the more light it will let through. Let’s start with a real basic standard lens and you can see what the difference is: A 28mm lens could have a number that says F2, or even F1.8, or F1.7. The F is what’s called the “f stop.” On your lens it says f/ then a number. That number basically is how open or closed the “aperture” of the lens is. It should say the f stop right on the side or the front of the lens. If we went from F2 to F1.4 (I think that is the next number sequence that was originally used), your lens would actually allow double the light through it. That starts letting you shoot your lens in lower light because it has the capability to let more light through it, OR allow you to take a photo in dimmer light. Now you see lenses announced that go down to F1.2 and that just gets even better. More options to shoot in lower light. As you will notice these two lenses I am showing above, they are very unique as they go to the limit of what has been produced so far. You will notice the Fuji Lens at the top is an F1.0 (they show it in a ratio number like this: 1:1, meaning the number after the colon, is it’s “speed” designation. The biggest problem you will run in to with using a lens this low (and the second lens above is an F .95 – even more light than the Fuji) is the amount of glass that has to be used. They are big in diameter, to allow more light through it. So these lenses are not cheap. The Fuji Lens above sells for around $1299.00. The Nikon lens, the 50mm F.95 sells for around $7999.00 US Dollars. Also, one more issue, is the depth of field is almost gone if you used your lens at F1. The wider the aperture the less depth of field you have on your photo. Or as I like to use as a reference: If you took a picture of a person’s face, the nose would be in focus but the ears would be out of focus. The choice is yours.
- There is usually always 1 more number on the lens, and I bring up the FujiFilm lens to show this number, and that is at the end of the description of the lens, you will see: 77. The little circle with a line through it is telling you that this is the filter size. That’s all! If you want to put on a filter, like a polarizing filter, or just a protective filter, you would need to purchase a size 77mm.
Whew!! That was deep, and I hope you understood all that. This is just something that a lot of people have no idea of what all these numbers mean, and I thought I would share that with you.