Why is this important. Well today I was looking to rent a lens for this weekend. I mainly want it for a shoot on Sunday at the train station in these photographs. We are doing a retro look with models dressed circa 1942. But Friday night we have a home football game. So I was thinking "hey I can get dual use out of this and get something that will rock the football game, and do a kicking review of it too". I started to look at the 70-200 and the 50-150mm f/2.8 lenses. I want to review both of these for you all. But then I started to wonder if they would work at the train station for a group of people. How to know unless I know the geek of focal lengths.
While I was there I decided to shoot some shots for this blog post to show the compression of images when moving from wide angle lenses. My telephoto is dead from taking a swim in the lake this summer, so I don't have anything longer than 55mm in the bag at the moment. I might just do a part two next week with a longer lens. Anyway, the first photo is taken at 18mm. This is not totally scientific, but it will give you a good idea what is going on. I set the tripod about 6 feet from the light. I put the outermost focal points in the middle plane on the outer edge of the bowl of the lights so I would have a reference point to frame up the lights the same every time. Looking to the left you can see all the way to the first window of the stationmasters office (if you are a train aficionado and I named that wrong please forgive me). Looking to the right you can see all the way to the edge of the house. We also see sky above and the tracks below.
So back to the focal length of the rental lens. If I opened the 18-55mm lens all the way to 18mm I could get the entire depot from about 12 feet away. So if I went to 24 feet I was shooting at around 36mm (remember this is not scientific - I was pacing this off for rough measurements). So first, this would put me way away from the models. Second, behind me was a cornfield. So I could only go so far back. When I went back to the edge of the field and went to 55mm I was losing parts of the front of the depot. So obviously the 70-200mm would be totally out for that shoot, and the 50-150mm lens would be a struggle. Now that I have figured this out I can get a good sense of what focal length to use when trying to capture a multi person scene shoot, and it is not a long telephoto lens at all.
The changes we are seeing between these photos are because of the change in the angle of view as we zoom in and out on the image. As you zoom out (like to 18mm) you will have a much wider angle of view right to left and top to bottom. As you zoom in, you will narrow that angle of view. This is the effect you get when people talk about a crop sensor body (a DX image sensor body) and how it makes a 50mm like a 75mm lens. You are not actually magnifying the image the same as a 75mm lens. But you are changing the angle of view, because you are cropping out the outer edges of the image. so it affects the overall image like putting on the longer focal length. One thing I am unsure of is the effect of greater magnification of using the 75mm on the FX body. All the websites talk about angle of view. But it would seem to me that the longer focal length would be magnifying the image optically too, giving a sharper image than the 50mm on the DX. I guess it is time to rent a D600 and make some comparisons for that. I just need to figure out how to get a good solid comparison. If anyone has any good information on this put it in the comments below!!
At any rate, we have seen how there are significant differences in the images between focal lengths. It is not just bringing something far closer. Also, you need to think about how much room you have between you and the subject, or how far back you might want to get from the subject with your lens choice too. Do you want to use a megaphone to give directions to your models? Or I suppose you could rent a Segway to go between the camera on a tripod and the models too.
I always highly recommend taking time to take a bunch of different test shots when trying to understand things. So for focal length go out and shoot a series of photos at different focal lengths yourself. To understand noise and high ISO take a series of shots at different ISOs and then look at them. The best way to understand shutter speed is do a series of photos of a waterfall from lightening fast all the way to at least a second or longer (hint: on the long shutter speeds use the self timer or a trigger to make sure you don't get blur from moving the camera when pushing the shutter). In a future post we will talk about that dreaded inverse square law and how it affects the light from your strobes or continuous lights.