images from artrenewal.org
I want to point out the difference between Constables paintings and those of the landscapists who preceded him. Above is a John Constable. It is naturalistic, that is, it is believable looking, rather than idealized and "mannered". Constable really got the look of nature in a way that preceding landscapists did not.
Above is a Claude Lorain. The earlier landscapists often saw the landscape in a classical manner. That was a traditional way of representing nature that relied as much on tradition as reference to actual vision. Constables paintings are made up of what earlier painters would have called splotches of paint, blots. He was showing the shimmering light reflecting from the things he was painting. The earlier painters painted the substance of the objects, Constable painted the effect of light upon them.
The branches of the trees in the Loraine seem leathery and look like that popular pattern of stoneware called willow. They reek of the studio, and sometimes look like they were done by a man who had heard about trees but had never actually seen one. Look at the tree in the Constable above and compare it with the Loraine below. All of the paintings on this page are wonderful art, but the Constables represent a new interest in presenting the actual appearance of nature.
Above, Salvatore Rosa. The earlier landscapists still thought of the landscape as a stage set that needed to be dignified by the presence of mythological figures or some storytelling aspect. This idea was reflected in Reynolds opinions on the hierarchy of painting., The paintings needed the figures and the narrative content to be high art. The Dutch didn't usually feel the need to do this though and that attitude was certainly a model for Constable.
Above is a Rubens. Rubens was a particular hero to Constable and he drew a lot of influence from him. The Rubens is still "classical" but they do have something in them that reminds me of Constable. Their handling is still "mannered", or executed in a standardized way rather than in clear imitation of nature. Constable remarked, that he saw "no handling" in nature.
Above is a Dutchman, Ruisdael. The area Constable painted, along the river Stour is in East Anglia, a low flat country very similar to that which the Dutch inhabited. There is a fondness for this low country amongst landscape painters and Seago worked this area too.