So you are look­ing for a new cam­era lens or you went out and bought a brand new D-SLR dig­i­tal cam­era with the kit lens. Now you want to pro­duce 11 x 14 or 16 x20 or larger sharp prints.  You will need a good tri­pod and know your cam­era lens sweet spot. The lens sweet spot is the aper­ture or F-stop set­ting which pro­duces sharpest image possible.

The lens sweet spot is deter­mined by which F-stop or aper­ture to get the sharpest image. All lenses have a sweet spot.  What we are going to deter­mine is what aper­ture will pro­duce the sharpest image.  A sim­ple rule of thumb is to take the widest aper­ture and stop it down 2 full f stops or 2 full aper­ture val­ues ( see chart below).  Let’s say you have a lens with a max­i­mum F-stop of F4.  That would make the sweet spot of your lens about F8.  The prob­lem is this may not be the exact sweet spot for your par­tic­u­lar lens. The only way to really know is to test the lens for your­self or you may find it in a lens review arti­cle in one of the many pho­tog­ra­phy mag­a­zines.  The prob­lem is that your lens may not have been tested or at least may not have iden­ti­fied the sweet spot of your lens.

Full F-stop Chart

Your lens may have other F-stops not listed on this chart above.

The chart above gives the cor­re­spond­ing aper­ture value to F-stop.  As each aper­ture value increases it cuts the light in half.  As each aper­ture val­ues decreases it dou­bles the light.

So let’s test the lens.  The first thing I did was find a test pat­tern chart to test the lens.  I found one at http://www.graphics.cornell.edu/~westin/misc/ISO_12233-reschart.pdf You can print it right from the site or down­loaded it.  Once down­loaded you can print the test chart in the best qual­ity your printer will allow you to print.   This is what you will need to run this test: your cam­era and lens; a sturdy tri­pod; a remote cable release; a board to mount the test chart on; the test chart.

Step 1. Attach the chart on the board with tape so that the chart will not move if there is any wind.

Step 2.  Find a bright spot out­side and set the board with the chart in the sun.

Step 3.  Install your cam­era on a tripod.

Step 4.  Select the cam­era to aper­ture priority.

Step 5.  Focus your cam­era on the chart.

Test Chart Loca­tion Below

Next, we’ll start with full open.  Let’s say at f5.6 and shoot your first pic­ture.  Next we’ll shoot a photo at each f-stop, mak­ing note of each photo’s f-stop so that when you import them you will know which F-stop cor­re­sponds to each pho­to­graph.  Be sure to use your remote shut­ter release cable and don’t move the cam­era through the whole process.  Please note  that any cam­era shake or move­ment will cause an inac­cu­rate test.

After you fin­ish, take your mem­ory card and import into your favorite photo edit­ing soft­ware.  (I use Adobe Light­room 2.)  Open the pho­tos in your edi­tor and set the  mag­ni­fi­ca­tion to one to one.  Use a side by side com­pare func­tion if avail­able.  Now, care­fully inspect and com­pare each pho­to­graph to see which one is the sharpest.  Once you have deter­mined which photo is the sharpest you have found your lens F-stop sweet spot.

If you are using a zoom lens you may want to repeat these steps for sev­eral focal lengths.  Let’s say you’re using a 70 to 200 mm lens.  First use 70mm then 130mm then 200mm. This will let you find the best F-stop for the sharpest pic­ture at all F-stops.

The chart below shows the tests results I found with the lenses I have.

Con­clu­sion.

One thing that I found when per­form­ing these tests is the bet­ter the lens qual­ity the harder it is to find the exact sweet spot. These lenses gen­er­ally had between two and three F-stops with the best sharp­ness. With some less expen­sive lens I found it easy to find a sin­gle F-stop sweet spot. But it is pos­si­ble with higher end con­sumer lenses with ED glass (extra low dis­tor­tion glass) you can get very sharp images.

Now you know what your lens sweet spot is.  Even if you have a less expen­sive lens, by using this knowl­edge, you will be able to get very sharp pho­tographs with most lenses.  But, we do live in the real world.  Some­times, because of low light con­di­tions, or the need to adjust the depth of field, we may have to adjust our F-stop to our needs.  So, if the sit­u­a­tion is right to use the sweet spot of your lens,  you will find you will get the sharpest pic­tures pos­si­ble with your lens.

The Tele­con­verter

I was not happy with the test results on the Nikon TC-20E II 2X Tele­con­verter.   I have read other reviews of the Nikon brand Tele­con­verter that come to same con­clu­sion. They also revealed that the Nikon TC-17E II 1.7x and Nikon TC-14E II 1.4x both had supe­rior per­for­mance over the TC-20E II 2X Teleconverter.

If you are in the mar­ket for a new lens or cam­era, buy the best lens you can afford.  In my opin­ion, the lens is more impor­tant than the cam­era.  Con­sider this the lens has a life cycle of about 10 or more years and because the cam­era tech­nol­ogy changes so fast, the camera’s life cycle is only 2 to 5 years.

So do your home­work before you buy any cam­era, lens or teleconverter.

Please share your ques­tions or com­ments below.

Happy sharp shooting.

Doug Nie­der­miller

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